Curing printed circuit board coatings

ABSTRACT

A method for curing a layer of liquid coating applied to surfaces of a printed circuit board is disclosed. The method comprises exposing the layer of liquid coating to a curing agent; and manipulating the printed circuit board during the exposure so as to prevent the liquid coating from flowing from its applied location in response to gravitational forces. An apparatus for implementing the above method is also disclosed. The apparatus comprises a motorized assembly for rotating the printed circuit board. The apparatus also comprises a controller that controls the motorized assembly to rotate the printed circuit board about one or more horizontal axes extending through a plane that includes the printed circuit board while a liquid coating applied to surfaces thereof is being cured.

RELATED APPLICATIONS

The present application is related to the following commonly-owned U.S.Patent Applications:

U.S. patent application Ser. No. 09/812,274 entitled “A BOARD-LEVEL EMISHIELD THAT ADHERES TO AND CONFORMS WITH PRINTED CIRCUIT BOARD COMPONENTAND BOARD SURFACES,” naming as inventors Samuel M. Babb, Lowell E. Kolb,Brian Davis, Jonathan P. Mankin, Kristina L. Mann, Paul H. Mazurkiewiczand Marvin Wahlen;

U.S. patent application Ser. No. 09/813,257 entitled “FILLER MATERIALAND PRETREATMENT OF PRINTED CIRCUIT BOARD COMPONENTS TO FACILITATEAPPLICATION OF A CONFORMAL EMI SHIELD,” naming as inventor Lowell E.Kol;

U.S. patent application Ser. No. 09/812,662 entitled “A LOW PROFILENON-ELECTRICALLY-CONDUCTIVE COMPONENT COVER FOR ENCASING CIRCUIT BOARDCOMPONENTS TO PREVENT DIRECT CONTACT OF A CONFORMAL EMI SHIELD,” namingas inventor Lowell E. Kolb;

U.S. patent application Ser. No. 09/974,375 entitled “A BOARD-LEVELCONFORMAL EMI SHIELD HAVING AN ELECTRICALLY-CONDUCTIVE POLYMER COATINGOVER A THERMALLY-CONDUCTIVE DIELECTRIC COATING,” naming as inventor PaulH. Mazurkiewicz;

U.S. patent application Ser. No. 09/974,367 entitled “A BOARD-LEVELCONFORMAL EMI SHIELD HAVING AN ELECTRICALLY-CONDUCTIVE POLYMER COATINGOVER A THERMALLY-CONDUCTIVE DIELECTRIC COATING,” naming as inventor PaulH. Mazurkiewicz; and

U.S. patent application Ser. No. 10/079,638 entitled “INTERFERENCESIGNAL DECOUPLING USING A BOARD-LEVEL EMI SHIELD THAT ADHERES TO ANDCONFORMS WITH PRINTED CIRCUIT BOARD COMPONENT AND BOARD SURFACES,”naming as inventor Lowell E. Kolb.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to managing signal interferenceon a printed circuit board and, more particularly, to systems andmethods for curing printed circuit board coatings.

2. Related Art

As large scale integrated circuits operate at increasingly higherspeeds, the need for devices to operate at faster switching rates hasincreased. As switching rates increase, problems which do not exist atlower speeds, such as shielding electronics from external sources ofelectromagnetic interference (EMI), become increasingly problematic.Moreover, maintaining sufficient electrical noise isolation and limitinginductance between components becomes more difficult as the frequency atwhich a circuit operates increases.

Conventionally, a shielded enclosure in the form of a metallic box orcage is placed around printed circuit board components to preventunwanted electromagnetic energy from impinging on the protectedcomponents. Such metallic enclosures have numerous drawbacks that limittheir shielding effectiveness. For example, electromagnetic energy oftenpenetrates the on-board metallic enclosure at gaps between the cage andthe printed circuit board surface. As a result, external electromagneticfields can be capacitvely (electrostatically) coupled onto traces on theprinted circuit board, magnetically coupled to conductive loops on theprinted circuit board, or electromagnetically coupled to conductorsacting as small antennas of electromagnetic radiation. In addition,power and signal lines are typically connected to interconnect postsmounted on the printed circuit board to receive power and transfercommunication signals. Interfering signals conducted along such powerand signal lines can enter the metallic enclosure.

SUMMARY OF THE INVENTION

In one aspect of the invention, a method for curing a layer of liquidcoating applied to surfaces of a printed circuit board is disclosed. Themethod comprises exposing the layer of liquid coating to a curing agent;and manipulating the printed circuit board during the exposure so as toprevent the liquid coating from flowing from its applied location inresponse to gravitational forces.

In another aspect of the invention, a method for adhering a conformalcoating applied to a printed circuit board is disclosed. The methodcomprises dispensing a liquid form of the conformal coating ontopredetermined regions of the printed circuit board; and rotating theprinted circuit board one or more revolutions about at least onehorizontal axis extending through the printed circuit board as thedispensed liquid coating is cured.

In a further aspect of the invention, an apparatus for manipulating aprinted circuit board is disclosed. The apparatus comprises a clampingdevice configured to removably secure the printed circuit board; amotorized assembly for rotating the clamping device; and a controllerthat controls the motorized assembly to rotate the printed circuit boardabout one or more horizontal axes extending through a plane thatincludes the printed circuit board while a liquid coating applied tosurfaces thereof is being cured.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention as well as thestructure and operation of various embodiments of the present inventionare described in detail below with reference to the accompanyingdrawings. In the drawings, like reference numerals indicate identical orfunctionally similar elements. Additionally, the left most one or twodigits of a reference numeral identify the drawing in which thereference numeral first appears. This description is given by way ofexample only and in no way restricts the scope of the invention. A briefdescription of the figures follows.

FIG. 1 is a cross-sectional view of one aspect of the conformal EMIshield of the present invention illustrating its conductive anddielectric coatings.

FIG. 2A is a side cross-sectional view of an integrated circuit mountedon a printed wiring board and covered with a conformal EMI shield inaccordance with one embodiment of the present invention.

FIG. 2B is a top cross-sectional view of the integrated circuitintroduced in FIG. 2A taken along section line I—I, showing only thedielectric coating portion of the conformal EMI shield of the presentinvention applied to the integrated circuit.

FIG. 2C is a top cross-sectional view of the integrated circuitillustrated in FIG. 2A taken along section line I—I, showing theconductive coating portion of the conformal EMI shield of the presentinvention applied over the dielectric layer shown in FIG. 2B.

FIG. 3 is a side cross-sectional view of a printed wiring board withvarious components mounted thereon with one embodiment of the conformalEMI shield illustrated in FIG. 1 applied thereto.

FIG. 4 is a cross-sectional view of a shielded connector such as thatshown in FIG. 3 with a ground moat mounted on the printed wiring boardthat surrounds the connector and is covered by the conformal EMI shieldof the present invention.

FIG. 5 is a cross-sectional view of a ground pad mounted on the printedwiring board and covered by the conformal EMI shield of the presentinvention.

FIG. 6A is a cross-sectional view of an edge region of a printed wiringboard showing a continuous conformal EMI shield of the present inventioncoating the top, edge and bottom surfaces of the printed wiring board.

FIG. 6B is a cross-sectional view of an edge region of a printed wiringboard showing the conformal EMI shield coating ground strips mounted onthe top and bottom surface proximate to the edge surfaces on which agrounded edge plating is mounted.

FIG. 6C is a cross-sectional view of an edge region of a printed wiringboard showing the conformal EMI shield coating ground strips mounted onthe top and bottom surface proximate to the edge surfaces with theground strips connected to a ground plane through ground vias.

FIG. 6D is a cross-sectional view of an edge region of a printed wiringboard showing the conformal EMI shield coating ground strips mounted onthe top and bottom surface proximate to the edge surfaces with a springclip electrically connecting the two ground moats.

FIG. 6E is a cross-sectional view of an edge region of a printed wiringboard showing the conformal EMI shield coating the top and bottomsurfaces with a spring clip electrically connecting the two conformalEMI shield regions.

FIG. 7 is a custom memory card coated with the conformal EMI shield inaccordance with one embodiment of the present invention.

FIG. 8A is a cross-sectional view of a printed wiring board with acomponent mounted thereon with a nonconductive component cover mountedover the component to encase the component in a compartment defined bythe cover and the printed wiring board.

FIG. 8B is a cross-sectional view of a printed wiring board with aprocessor mounted thereon with a nonconductive, conformal cover with acontoured, arbitrary shape mounted over the processor to encase theprocessor in a compartment defined by the cover and the printed wiringboard.

FIG. 8C is a cross-sectional view of the printed wiring board andcomponent compartment shown in FIG. 8B with a dielectric coating of thepresent invention covering the surface of the component cover andsurrounding printed wiring board.

FIG. 8D is a cross-sectional view of the printed wiring board andcomponent compartment shown in FIG. 5C with a conductive coating of thepresent invention covering the dielectric coating, forming conformal EMIshield of the present invention.

FIG. 8E includes two figures, FIGS. 8E-1 and 8E-2.

FIG. 8E-1 is a cross-sectional view of the component cover shown in FIG.8A illustrating one embodiment of a line or severability in the form ofa crease.

FIG. 8E-2 is a cross-sectional view of the component cover shown in FIG.8A illustrating an alternative embodiment of a line or severability.

FIG. 9A is a cross-sectional view of a printed circuit board with afiller material applied to certain regions thereof in accordance withone embodiment of the invention to cover, encapsulate enclose orotherwise coat cavities on the printed circuit board, such as betweenthe components and printed wiring board.

FIG. 9B is a top perspective view of a void formed in the fillermaterial shown in FIG. 9A.

FIG. 9C is a cross-sectional view of a printed circuit board with thefiller material applied thereto, as shown in FIG. 9A, with thedielectric coating of the present invention applied to the surface ofthe filler material and neighboring printed wiring board surfaces.

FIG. 9D is a cross-sectional view of the printed circuit board with afiller material and the dielectric coating applied thereto, as shown inFIG. 9C, with the conductive coating of the present invention applied tothe surface of the dielectric coating to form the conformal EMI shieldof the present invention.

FIG. 10 is a flow chart of the operations performed to manufacture anEMI-shielded printed circuit board in which component covers and fillermaterial are utilized with the conformal EMI shield in accordance withone embodiment of the present invention.

FIG. 11 is a flow chart of the primary operations performed in utilizinga component cover shown in FIGS. 8A–8E with the conformal EMI shieldintroduced in FIG. 1.

FIG. 12 is a scale illustrating the relative electrical conductivity ofintrinsically conductive polymers (ICPs), metal conductors,semi-conductors and insulators.

FIG. 13 includes three figures, FIGS. 13A, 13B and 13C.

FIG. 13A illustrates the state of one embodiment of the conductivepolymeric dispersion when applied to the surface of a printed wiringboard or component mounted thereon.

FIG. 13B illustrates the state of the embodiment of the conductivepolymeric dispersion illustrated in FIG. 13A after curing to form anelectrically conductive polymeric coating adhered to the surface of theprinted wiring board or component to which it was applied.

FIG. 13C is an illustration of the conductive polymeric dispersion ofFIG. 13B in operation conducting electricity across the surface of theprinted wiring board or component to which it was applied.

FIG. 14 is a scale illustrating the relative thermal conductivity ofacrylics/urethanes, metals and thermal loading materials utilized in oneaspect of the present invention.

FIG. 15 includes two figures, FIGS. 15A and 15B.

FIG. 15A illustrates the state of one embodiment of the thermallyconductive dielectric dispersion when applied to the surface of aprinted wiring board or component mounted thereon.

FIG. 15B illustrates the state of the embodiment of the thermallyconductive dielectric dispersion of FIG. 15A after curing to form athermally conductive dielectric to coating adhered to the surface of aprinted wiring board or component.

FIG. 16A is a side cross-sectional view of a decoupling circuit inaccordance with one embodiment of the present invention.

FIG. 16B is the same cross-sectional view as that illustrated in FIG.16A with the receiver loop formed by the electrical filter circuitportion of the decoupling circuit emphasized.

FIG. 16C is the same cross-sectional view as that illustrated in FIGS.16B and 16C with the noise suppressor receiver loop portion of thedecoupling circuit emphasized.

FIG. 16D is the same cross-sectional view as that illustrated in FIGS.16B–16D showing the flow of current induced in the filter receiver loopand noise suppressor receiver loop.

FIG. 17 is a front cross-sectional view of the decoupling circuitillustrated in FIGS. 16A–16D illustrating two such circuits adjacent toeach other and having the electrical filter portions thereof encasedwithin a grounded compartment.

FIG. 18A is a top view of a printed circuit board illustrating the topsurfaces of the ground lands, interconnect post and surface-mountedcapacitor of the electrical filter illustrated in FIGS. 16A–16D and 17.

FIG. 18B is a top view of the printed circuit board shown in FIG. 18Asubsequent to the application of the dielectric coating of the conformalEMI shield.

FIG. 18C is a top view of the printed circuit board shown in FIGS. 18Aand 18B subsequent to the application of the conductive coating of theconformal EMI shield to the printed circuit board surfaces illustratedin FIG. 18B.

FIG. 19 is a perspective view of a curing unit for use in a processingline for manufacturing printed circuit boards in accordance with oneembodiment of the present invention.

FIG. 20 is a perspective view of a rotation cage suitable for rotating acoated printed circuit board while the coating is cured in accordancewith one embodiment of the present invention.

FIG. 21 is a perspective view of a robotic arm suitable for mounting inthe curing equipment illustrated in FIG. 19 in accordance with oneembodiment of the present invention.

DETAILED DESCRIPTION

1. Introduction

The present invention is directed to an electrically continuous,grounded conformal electromagnetic interference (EMI) protective shield,methods for applying the same directly to the surfaces of a printedcircuit board, and a printed circuit board coated with such a conformalEMI shield. The conformal EMI shield of the present invention adheres toand conforms with the surface of the components and printed wiring boardto which it is applied. Because the conformal EMI shield is relativelythin, the conformal EMI shield takes the shape of the covered componentswithout changing significantly the dimensions of the printed circuitboard region to which it is applied. The conformal EMI shield of thepresent invention includes two primary coatings. A conductive coatingprevents electromagnetic radiation from passing through the conductivecoating, whether generated by the shielded components or emanating froma source not on the printed circuit board. The conformal EMI shield alsoincludes a dielectric coating interposed between the conductive coatingand the printed circuit board to prevent the conductive coating fromelectrically contacting predetermined portions of the coated printedcircuit board region.

Advantageously, the conformal EMI shield of the present inventioncompletely and contiguously coats the printed circuit board; that is,there are no gaps, voids or breaks in the conformal EMI shield. Nor arethere any gaps, breaks for voids between the conformal shield and thecoated surfaces. This enables the conformal EMI shield to providesignificantly improved shielding effectiveness as compared withconventional shielding techniques. In contrast to such approaches, thereare no opportunities for EMI to penetrate the conformal EMI shield, anoccurrence experienced by conventional approaches due to the noted gapbetween the metallic cages and printed wiring board surface.

In one particular aspect of the invention described below, a decouplingcircuit that attenuates conducted and induced interfering signalstraveling or appearing on power and signal lines connected to a printedcircuit board, while shielding the printed circuit board fromelectromagnetic interference (EMI). The decoupling circuit includes anelectrical filter circuit electrically coupled between a ground land andan interconnect post to which power or signal lines are connected.Generally, the electrical filter passes transmitted signals(s) whileblocking conducted interfering signal(s) traveling through theinterconnect post. A receiver loop is formed by the electrical filtercomponent(s), power/signal post, ground land, ground via and groundplane, and interconnecting surface traces. Unfortunately, fields can becapacitively, magnetically and/or electromagnetically coupled to all ora portion of this receiver loop, referred to herein as the filterreceiver loop.

The portion of the printed circuit board covered with the conformal EMIshield includes the decoupling circuit. Here, the EMI-protective coatingis conformally secured to surfaces of the electrical filter, ground landand interconnecting surface traces. The conductive coating of the EMIshield is electrically connected to the ground land and electricallyinsulated from the filter components and surface traces. The EMI shieldconductive coating, ground land, electrical filter components andinterconnecting surface traces together form a noise suppressor receiverloop. In such a configuration, the noise suppressor receiver loop andthe filter receiver loop share an electrical path between theinterconnect post and ground land. The noise suppressor receiver loopgenerates a current the cancels interfering signals magnetically andelectromagnetically generated in the filter receiver loop, while theconductive coating attenuates electrostatically coupled fields.

Thus, the decoupling circuit reduces interfering signals that can enterelectrical systems through power-line inputs or through signal input andoutput lines while simultaneously reducing the effects of interferingsignals generated as a result of electrostatic, magnetic andelectromagnetic coupling of a filter receiver loop.

2. Conformal EMI Shield Materials

-   -   A. Overview

As noted, the conformal EMI shield includes a conductive coating and adielectric coating permanently bonded to each other. The materials thatcan be used in the conductive and dielectric coatings are describedbelow with reference to FIGS. 1–3. FIG. 1 is a cross-sectional view ofone embodiment of the conformal EMI shield of the present invention.FIG. 2A is a cross-sectional view of an integrated circuit componentmounted on a printed wiring board forming a portion of a printed circuitboard. The integrated circuit component and printed wiring board havebeen coated with one embodiment conformal EMI shield of the presentinvention. FIG. 2B is a top view of the integrated circuit componentillustrated in FIG. 2A taken along section line I—I illustrating theapplication of the shield's dielectric coating in accordance with oneembodiment of the present invention. FIG. 2C is a top view of theintegrated circuit component taken along the same section lineillustrating the application of the conformal EMI shield's conductivecoating in accordance with one embodiment of the present invention.

Referring now to FIG. 1, this embodiment of EMI shield 100 includes adielectric coating 102 and a conductive coating 104. The exposedsurfaces of selected printed circuit board regions 106 are coated withconformal EMI shield 100. Such surfaces can be, for example, the top,side and, if exposed, bottom surface of a component, the surface of anyleads, wires, etc, that are connected to the component, as well as anyother exposed surface of any other portions, elements, sections orfeatures (hereinafter “features”) of the components and printed wiringboard located in the coated printed circuit board region. It should beappreciated that the selection of the combination of material propertiesfor dielectric coating 102 and conductive coating 104 is important toachieving a conformal EMI shield that can be applied directly to printedcircuit board surfaces without damaging components and connections, thatdoes not expose the coated regions to risk of electrical shorts, andthat completely envelops or encases the coated regions to provide adesired shielding effectiveness. As will be described in detail below,conformal EMI shield 100 not only achieves such operational objectives,but does so, as noted, by directly coating; that is, physically adheringto, the surface of coated printed circuit board regions. This enablesconformal EMI shield 100 to completely and conformingly coat thesurfaces of the shielded printed circuit board regions.

-   -   B. Dielectric Coating

Dielectric coating 102 is comprised of a material that is electricallynonconductive and, preferably, thermally conductive. Importantly, thematerial properties of dielectric coating 102, described in detailbelow, enable dielectric coating 102 to completely coat and securelyattach to component and board surfaces to which it is applied.Generally, the material properties of dielectric coating 102 includeprimarily a combination of viscosity and adhesion sufficient to enabledielectric coating 102 to be applied via atomization spray techniquesand, once applied, to adhere to the surface in the immediate vicinity ofwhere it was applied. In other words, adhesiveness of dielectric coating102 is sufficient to prevent dielectric coating 102 from separating fromthe surface to which it is applied prior to curing, a phenomenoncommonly referred to as dewetting. Such a condition will otherwiseresult in a void in dielectric coating 102, providing the potential ofan electrical short in the exposed portion of printed wiring board orcomponent 106. Dielectric coating 102 can comprise multiple,successively applied layers of dielectric material. As such, dielectricmaterial 102 preferably also includes the properties necessary to enableit to adhere to or bond with previously applied dielectric layers.

Specifically and in one embodiment, dielectric coating 102 has aviscosity of at least 45″ #2 Zahn Cup (full body). In anotherembodiment, dielectric coating 102 has a viscosity in the range of50–100″ #2 Zahn Cup (full body). In one preferred embodiment, dielectriccoating 102 has a viscosity of 70–95″ #2 Zahn Cup (full body). Adielectric coating 102 having any of the above viscosity values can beapplied uniformly using a conventional spray atomization technique. Thisenables dielectric coating 102 to completely access and coat thesurfaces of the components and board that are located underneathcomponent leads, between components and wiring board surfaces and otherregions that are exposed yet difficult to access. Such features of theprinted circuit board are referred to generally herein as cavities. Ingeneral, dielectric coating 102 can adhere to the materials utilized inthe printed circuit board. Such materials include, but are not limitedto, FR-4 such as polymethylmethacrylates, bisphenol-A based epoxy andfiberglass, ceramics such as aluminum oxide and silicon dioxide,silicon, polyimide (silicon wafers), polyethylene (sockets),polyethylene terephthalate, polystyrene (sockets), polyphenylsulfone orPPS (chip sockets), polyvinyl chloride or PVC (wire coverings), siliconerubbers such as RTV (various surfaces), aluminum, gold, stainless steeland low carbon steel), tin, lead, and others. Dielectric coating 102preferably has an adhesion that enables it to pass the ASTM D-3359-83Method A Tape Test using a 1″ (25 mm wide) semi-transparentpressure-sensitive tape with and adhesion strength of 25–70 and, morepreferably, 30–50 ounces per inch when tested in accordance with ASTMTest Method D-3330.

In one embodiment, dielectric coating 102 is comprised primarily ofClear Water Reducible Barrier Coat, Formula Number CQW-L200DF,manufactured by The Egyptian Coating Lacquer Manufacturing Company,Franklin, Tenn., USA. CQW-L200DF has a viscosity in the range of 50–60″#2 Zahn Cup (full body) and an adhesion that enables it to pass the ASTMD-3359-83 Method A Tape Test using a 1″ (25 mm wide) semi-transparentpressure-sensitive tape with an adhesion strength of 40±2.5, ounces perinch when tested in accordance with ASTM Test Method D-3330. CQW-L200DFprovides excellent adhesion to materials commonly found on a printedcircuit board comprising, but not limited to, the materials noted above.

Non-electrical-conductive conformal coatings have minimal thermalcharacteristics due to their low density, molecular properties, etc.Printed circuit boards that are coated with such conformal coatingscould, under certain circumstances, overheat. To prevent suchoccurrences, it is preferred that when used in accordance with theconformal EMI shield 100 of the present invention, dielectric coating102 is thermally conductive. In accordance with one embodiment of thepresent invention, dielectric coating 102 is doped or loaded with anon-electrically conductive, dense substance having relatively improvedthermal transfer characteristics, referred to herein as a thermalloading material. The resulting dielectric coating 102 is referred toherein as a thermally conductive dielectric coating.

FIG. 14 is a scale illustrating the relative thermal conductivity ofthermal loading materials, polymers and metal conductors. In thisillustrative scale, thermal conductivity is presented in units of Wattsper millikelvin (W/mK). The vertical column in the middle of the figuresets forth a thermal conductivity scale from 0 through 180 W/mK. On theleft-hand side of the scale are insulators such as acrylics andurethanes 1402, and aluminum alloy 2024 (reference numeral 1404).Acrylics and urethanes 1402 have a thermal conductivity of 0.06 W/mK.Aluminum alloy 1404, a popular alloy used in heat sinks, has a thermalconductivity of 130 W/mK.

For ease of comparison, thermal loading materials 1400 are positioned onthe right side of the scale. Examples of thermal loading materials 1400illustrated in FIG. 14 include aluminum oxide (AlO₃) 1410, magnesiumoxide (MgO) 1408 and boron nitride (BN) 1406. As shown, the conductivityof aluminum oxide 1410 is 30 W/mK while the thermal conductivity ofmagnesium oxide is 36 W/mK. Both of these materials have a thermalconductivity that is substantially greater than acrylics and urethanes1402, with a thermal conductivity of 0.06 W/mK. Boron nitride 1406 has athermal conductivity of 160.6 W/mK, making it a preferred thermalloading materials 1400. As shown, the thermal conductivity of boronnitride 1406 is greater than that of even aluminum alloy 1404, a metalused specifically to conduct heat away from heat-generating components.It should be understood, however, that in contrast to such metallicmaterials, boron nitride 1406 is non-electrically conductive, as are allthermal loading materials 1400 as defined herein.

FIG. 15 includes 2 figures illustrating the state of one preferredembodiment of a thermally conductive dielectric dispersion 1500 whenapplied (FIG. 15A) and cured (FIG. 15B) in accordance with the teachingsof the present invention. Once applied and cured, thermally conductivedielectric coating 1502 is a substantially dry, solid coating. However,the state of thermally conductive dielectric coating 1502 prior to whenit is applied to the printed circuit board can vary. Preferably,thermally conductive dielectric coating 1502 is applied usingconventional spray atomization techniques. To facilitate theimplementation of such application techniques, it is preferred that thethermally conductive dielectric coating 1502 is provided in the form ofa dispersion, referred to herein as a thermally conductive dielectricdispersion 1500.

In FIG. 15A, thermally conductive dielectric dispersion 1500 is shownafter it has been applied to a surface of a printed circuit board orcomponent 106 but before it is cured. In this illustrative embodiment,thermally conductive dielectric dispersion 1500 includes a bindermaterial 1504 and thermal loading material 1500 suspended in a baseliquid 1506. Binder material 1504 can be, for example, any well-knownand commercially available acrylic or urethane.

As one of ordinary skill in the art would find apparent, thermallyconductive dielectric dispersion 1500 is a heterogeneous solution inwhich thermal loading material 1400 is dispersed in a base liquid 1506such as water or organic solvent. As the contiguity of the thermalloading material 1400 in the cured dielectric coating 1502 increases, sotoo does the ability of dielectric coating 1502 to conduct heat.Accordingly, it is preferable that the suspension is substantiallyuniform to insure the contiguity of the thermal loading material 1400 inthe resulting dielectric coating 1502.

With regard to base liquid 1506, waterborne dispersions are preferredbecause they are substantially easier to process than dispersions usingorganic solvents. In addition, the use of water eliminates theenvironmental and processing drawbacks associated with the use oforganic solvent emissions. However, organic solvents including, forexample, N-Methyl-Pyrolidinone (NMP), various alcohols, acetone,Methyl-Ethyl-Ketone (MEK), and others, may be a suitable base liquid1506 in certain applications.

In one embodiment, thermally conductive dielectric dispersion 1500 isformed by doping or loading a conformal coating dispersion such as acommercially-available acrylic or urethane dispersion with a thermalloading material 1400. Prior to doping, such acrylic or urethanedispersions are referred to herein as an intermediate dispersion. Suchintermediate dispersions have a binder 1504 of either acrylic orurethane, and a base liquid 1506 of water or organic solvents.

In one embodiment, such acrylic intermediate dispersions include, forexample, the waterborne LOCTITE® product 394 Shadowcure™ urethaneacrylate conformal coating available from the Loctite Corporation, RockyHill, Conn., which has a thermal conductivity of approximately 0.16 W/mKwhen measured in accordance with ASTM F-433. Another acrylicintermediate dispersion is the waterborne LOCTITE® product 397Shadowcure™ urethane acrylate conformal coating which has an ASTM F-433thermal conductivity of approximately 2.17 W/mK. These intermediatedispersions serve as excellent dielectrics. For example, at 1 kHz,Product 394 has a dielectric constant & loss of 3.3 and 0.015; at 1 MHz,2.9 and 0.020, when measured in accordance with ASTM D150. Product 394has a volume resistivity of 3.8×10¹⁶ ohm-cm and a surface resistivity of7×10¹⁶ ohms when measured in accordance with ASTM D257. At 1 kHz,product 397 has a dielectric constant & loss of 4.6 and 0.045; at 1 MHz,3.8 and 0.048, when measured in accordance with ASTM D150. Product 397has a volume resistivity of 3.17×10¹⁵ ohm-cm and a surface resistivityof 2.36×10¹⁶ ohms when measured in accordance with ASTM D257. TheTechnical Data Sheets for these two intermediate dispersions can beobtained from Loctite Corporation (www.loctite.com).

Another embodiment of an acrylic intermediate dispersion suitable foruse in the present invention is the waterborne HumiSeal® 1B12 or 1B31acrylic conformal coatings available from HumiSeal Corporation,Woodside, N.Y. These two products also serve as good dielectrics and canbe doped with thermal loading materials 1400. For example, the HumiSeal®1B12 has a dielectric constant of 2.8 and surface resistivity of250×10¹² ohms when measured in accordance with ASTM D257. The HumiSeal®1B31 has a dielectric constant of 2.5 and surface resistivity of800×10¹² ohms when measured in accordance with ASTM D257. The TechnicalData Sheets for these two intermediate dispersions can be obtained fromHumiSeal Corporation (www.humiseal.com).

As noted, intermediate dispersions can also have a urethane binder 1504.In one embodiment, such an intermediate dispersion is the above-notedClear Water Reducible Barrier Coat, Formula Number CQW-L200DF,manufactured by The Egyptian Coating Lacquer Manufacturing Company. Thisintermediate dispersion has a water base liquid 1506 and a urethanebinder 1504. The Technical Data Sheets for this intermediate dispersioncan be obtained from The Egyptian Coating Lacquer Manufacturing Company(www.egyptcoat.com).

An alternative intermediate dispersion with a urethane binder 1504 isthe waterborne HumiSeal® 2A64 urethane conformal coatings available fromHumiSeal Corporation. This intermediate dispersion has a water baseliquid 1506 and a urethane binder 1504. The HumiSeal® 2A64 has adielectric constant of 3.5 and surface resistivity of 250×10¹² ohms whenmeasured in accordance with ASTM D257. The Technical Data Sheet for thisintermediate dispersion can also be obtained from HumiSeal Corporation.

The thermal loading material can be any non-electrically conductive,highly thermally conductive material having a thermal conductivity ofgreater than 20 W/mK. Preferably, the thermal conductivity of thethermal loading material is greater than 30 W/mK. In still otherpreferred embodiments, the thermal conductivity of the thermal loadingmaterial is greater than 100 W/mK. Examples of thermal loading material1400 were provided above with reference to FIG. 14, namely, boronnitride (BN) 1406, aluminum oxide (AlO₃) 1408 and magnesium oxide (MgO)1410. It should be apparent to those of ordinary skill in the art thatother thermal loading materials 1400 now or later developed can beutilized.

In one particular embodiment, thermal loading material 1400 is boronnitride. In some embodiments such as those in which boron nitride isadded to an intermediate dispersion, the boron nitride is provided inpowder form. Boron nitride is a man-made ceramic having highlyrefractory qualities with physical and chemical properties similar tocarbon. In one embodiment, thermal loading material 1400 is agraphite-like boron nitride (g-BN), more commonly referred to ashexagonal boron nitride (h-BN). In another embodiment, thermal loadingmaterial 1400 is a cubic boron nitride (c-BN), more commonly referred toas diamond Boron Nitride. H-BN has soft, lubricious qualities while c-BNis hard and abrasive. Specific examples of boron nitride include themany of the coarse and file mesh, high and low density CarboTherm™ BNpowders available from Carbonundum Corporation, Amherst, N.Y.(www.carbon.com) (CarboTherm is a trademark of Carbonundum Corporation).In other embodiments, one or more of the many grades of Boron Nitrideavailable from Advanced Ceramics Corporation, Cleveland, Ohio(www.advceramics.com) can be used.

As noted, thermal loading material 1400 can also be aluminum oxide(Al₂O₃). In one embodiment, the aluminum oxide thermal loading material1400 is the Aldrich product 23,474-5 aluminum oxide powder availablefrom the Sigma-Aldrich Company, Milwaukee, Wis. Other aluminum oxidepowders could also be used depending on the desired characteristics ofthe thermally conductive dielectric coating 1502. In one embodiment, thegrade of aluminum oxide powder is between 10–5000 mesh. The TechnicalData Sheets for these and other aluminum oxide powders are availablefrom Sigma-Aldrich (www.sigma-aldrich.com).

The other noted thermal loading material 1400 was magnesium oxide (MgO).In one embodiment, the magnesium oxide thermal loading material 1400 isthe Aldrich product 342815 fused magnesium oxide having a 150–325 meshand an assay of 95%. In another embodiment, the magnesium oxide thermalloading material 1400 is the Aldrich product 342823 fused magnesiumoxide having a 40 mesh and an assay of 90%. In further embodiment, themagnesium oxide thermal loading material 1400 is the Aldrich product342777 fused magnesium oxide chips having a −4 mesh and an assay of99.9%. In a still further embodiment, the magnesium oxide thermalloading material 1400 is the Aldrich product 342785 fused magnesiumoxide pieces having a 3–12 mm size and an assay of 99.95%. Othermagnesium oxide powders could also be used depending on the desiredcharacteristics of the thermally conductive dielectric coating 1502. TheTechnical Data Sheets for these and other magnesium oxide powders areavailable from Sigma-Aldrich.

Following are six categories of exemplary formulations for thermallyconductive dielectric dispersion 1500. Each example formulation providesa range of percentages (by weight) for each component.

Example 1: Boron Nitride & Acrylic Dispersion 1500 Thermal loadingmaterial 1400: 10%–80% BN, 0.1–10 micron powder Binder 1504: 90%–20%Acrylic Base Liquid 1506: water or organic solvent Curing: UV orthermally curedThe binder 1504 and base liquid 1506 can be provided in the above-notedacrylic intermediate dispersions. In accordance with the presentinvention, such intermediate dispersions are doped with the specifiedthermal loading material 1400 to form one embodiment of thermallyconductive dielectric dispersion 1500.

Example 2: Boron Nitride & Urethane Dispersion 1500 Thermal loadingmaterial 1400: 10%–80% BN, 0.1–10 micron powder Binder 1504: 90%–20%Urethane Base Liquid 1506: water or organic solvent Curing: UV and/orthermally curedThe binder 1504 and base liquid 1506 can be provided in the above-notedurethane intermediate dispersions. In accordance with the presentinvention, such intermediate dispersions are doped with the specifiedthermal loading material 1400 to form one embodiment of thermallyconductive dielectric dispersion 1500.

Example 3: Aluminum Oxide & Acrylic Dispersion 1500 Thermal loadingmaterial 1400: 10%–80% Al₂O₃, 100 mesh, 99% corundum, alpha-phase Binder1504: 90%–20% Acrylic Base Liquid 1506: water or organic solvent Curing:UV or thermally curedThe binder 1504 and base liquid 1506 can be provided in the above-notedacrylic intermediate dispersions. In accordance with the presentinvention, such intermediate dispersions are doped with the specifiedthermal loading material 1400 to form one embodiment of thermallyconductive dielectric dispersion 1500.

Example 4: Aluminum Oxide & Urethane Dispersion 1500 Thermal loadingmaterial 1400: 10%–80% Al₂O₃, 100 mesh, 99% corundum, alpha-phase Binder1504: 90%–20% Urethane Base Liquid 1506: water or organic solventCuring: UV and/or thermally curedThe binder 1504 and base liquid 1506 can be provided in the above-notedurethane intermediate dispersions. In accordance with the presentinvention, such intermediate dispersions are doped with the specifiedthermal loading material 1400 to form one embodiment of thermallyconductive dielectric dispersion 1500.

Example 5: Magnesium Oxide & Acrylic Dispersion 1500 Thermal loadingmaterial 1400: 10%–80% MgO, 150 mesh Binder 1504: 90%–20% Acrylic BaseLiquid 1506: water or organic solvent Curing: UV or thermally curedThe binder 1504 and base liquid 1506 can be provided in the above-notedacrylic intermediate dispersions. In accordance with the presentinvention, such intermediate dispersions are doped with the specifiedthermal loading material 1400 to form one embodiment of thermallyconductive dielectric dispersion 1500.

Example 6: Magnesium Oxide & Urethane Dispersion 1500 Thermal loadingmaterial 1400: 10%–80% MgO, 150 mesh Binder 1504: 90%–20% Urethane BaseLiquid 1506: water or organic solvent Curing: UV and/or thermally curedThe binder 1504 and base liquid 1506 can be provided in the above-notedurethane intermediate dispersions. In accordance with the presentinvention, such intermediate dispersions are doped with the specifiedthermal loading material 1400 to form one embodiment of thermallyconductive dielectric dispersion 1500.

It should be appreciated that the above formulations are exemplary only.For example, as noted above, that there are many variations in thecharacteristics of binder 1504 and the intermediate dispersion in whichit is suspended, as well as the thermal loading material 1400. Thecharacteristics of the resulting thermally conductive dielectric coating1502 will vary according to the selected combination of properties. Inaddition, thermally conductive dielectric dispersion 1500 can includeother components. Such other components can include materials tofacilitate a particular process or to alter a particular characteristic.

The utilization of a thermal loading material increases the thermaltransfer ability of dielectric coating 102 to the point whereoverheating of the underlying printed circuit board components iseliminated. Thus, it should be appreciated that thermally conductiveconformal dielectric coating 1502 can have applications beyond conformalEMI shield 100. For example, dielectric coating 102 can be applied aloneto a printed circuit board. In such an application dielectric coating1502 can provide protection against adverse environmental effects suchas humidity, salt air and the like while not causing a significant risein the temperature of the components on the printed circuit board. Asnoted, thermally conductive dielectric coating 1502 is one preferredembodiment of dielectric coating 102. As such, dielectric coating 102 isgenerally referenced below.

Referring now to FIG. 15B, the state of thermally conductive dielectricdispersion 1500 after curing is a contiguous solid adhered to thesurface of printed wiring board or component 106, referred to as athermally conductive dielectric coating. As shown in FIG. 15B, the baseliquid 1506 in which the solids were borne is removed during curing.When cured, binder 1504 binds together to form a contiguous, rigidsurface in which thermal loading material 1400 is suspended, forming athermally conductive dielectric coating 1502 secured to wiring board orcomponent surface 106.

As is well-known in the art, thermally conductive dielectric dispersion1500 can include components that facilitate a desired curing process.For example, in one embodiment, thermally conductive dielectricdispersion 1500 can be UV cured. In such an embodiment, photosensitizingagents such as a UV-curable acrylic can in included in thermallyconductive dielectric dispersion 1500. Alternatively, certainembodiments of the thermally conductive dielectric dispersion 1500 canbe heat cured. In such embodiments, thermally conductive dielectricdispersion 1500 includes a heat curing agent such as an anhydride. Inone embodiment, additional materials are added to thermally conductivedielectric dispersion 1500 to facilitate both temperature and UV curing.For example, in one embodiment, thermally conductive dielectricdispersion 1500 includes the Shadowcure® material available from theLoctite Corporation, Rocky Hill, Conn.

Shadowcure includes both photosensitizing and heat curing agents.Shadowcure enables® to cure in response to both exposure to UV light andtemperature. This embodiment is desirable in those applications in whichthe printed circuit board configuration is such that there are smallgaps or spaces between component leads, neighboring components andbetween components and the surface of the printed wiring board. Thesevarious spaces are referred to herein generally and collectively as“cavities.” In such applications, UV light may not be able to impingeupon the dielectric coating 102 located in such cavities. As a result, aUV curing process may only result in the curing of that portion of thedispersion applied to openly exposed surfaces. However, the printedcircuit board can then be heat treated to cure the remaining portions ofdielectric coating 102.

In certain applications, there may be surfaces on printed circuit board304 that are more difficult to adhere to despite dielectric coating 102having a combination of properties noted above. In particular, cavitiesand very sharp or pointed surfaces provide less opportunity for amaterial to adhere to the defining surfaces. In such applications, it ispreferred that a conservative approach is taken with regard to coveragesince incomplete coverage of the printed circuit board can lead to anelectrical short circuit when conductive coating 104 is applied.Accordingly, in such applications, dielectric coating 102 can be appliedin multiple applications, each resulting in a layer of dielectricmaterial coating the covered region of the printed circuit board. Forexample, when implementing any of the above embodiments of dielectriccoating 102, it is preferred that dielectric coating 102 is applied intwo applications of approximately 1 mil each, for a total thickness ofapproximately 2 mils. Alternative embodiments have a dielectric coatingthickness 1.5–2.5 mils; and 2–4 mils. Each layer is preferably appliedwith 4 or 5 cross-coats, with a delay or pause between the first andsecond applications of approximately 1 to 2 minutes to allow the firstlayer to set up before the second layer is applied.

In such embodiments, the initial layer may have a void located at theapex of a sharp edge or within a cavity. Each subsequent cross-coat ofdielectric coating 102 adheres to the prior layer as well as theunderlying printed circuit board surface, reducing the size of the void.Ultimately, the void is filled or eliminated with a subsequentcross-coat or layer of dielectric material. As is well-known in therelevant arts, cross-coats are implemented to insure uniform applicationof dielectric coating 102 when each layer of dielectric coating 102 isapplied manually. However, such cross-coats are not necessary whendielectric coating 102 is applied with robotic or other automatedequipment. The temperature at which dielectric coating 102 is appliedcan vary depending on the selected embodiment. For example, certainembodiments of dielectric coating 102 is applied at room temperature,between 60–100 degrees Fahrenheit, although other applicationtemperatures may be specified by the manufacturer of the thermallyconductive dielectric coating components. Accordingly, it should beapparent to those of ordinary skill in the art that the applicationtemperature as well as other aspects of the manufacturing process willvary with the composition of dielectric coating 102 and, in general, theapplication.

Although dielectric coating 102 can be cured at room temperature, toexpedite manufacturing processes and to remove any water-basedcomponents from dielectric coating 102, dielectric coating 102 ispreferably thermally cured at an elevated temperature below that whichthe underlying printed circuit board can withstand. It should beapparent to those of ordinary skill in the art that dielectric coating102 need only be cured to the extent necessary to apply conductivecoating 104. As will be described below, both dielectric coating 102 andconductive coating 104 are thermally cured after conductive coating 104is applied.

It should be understood that the thickness of dielectric coating 102 candiffer from that noted, depending on the application. For example, in analternative embodiment, dielectric coating 102 is formed with 2 to 4cross-coats for each of 4 layers of dielectric material, resulting in athickness of approximately 6 to 10 mils. Thus, dielectric coating 102has a combination of adhesion and viscosity that enables it to form auniform, contiguous surface over the coated surfaces with no voidsformed therein.

An example of dielectric coating 102 applied to the integrated circuitshown in FIG. 2A is illustrated in FIG. 2B. As shown therein, dielectriccoating 102 adheres to the entire exposed surface of integrated circuitleads 208, including those lead surfaces that are adjacent to and facingthe side surface of integrated circuit 204. In addition, dielectriccoating 102 coats the side surfaces of integrated circuit body 204 thatare accessible only through gaps between neighboring leads 208. Notethat the thickness of dielectric coating 102 may vary slightly, beinggreater where access is more direct. Nevertheless, dielectric coating102 completely coats the entire exposed surface of integrated circuit204; that is, there are no voids, gaps, breaks or spaces in dielectriccoating 102.

-   -   C. Conductive Coating

As noted, conductive coating 104 is the outer coating of conformal EMIshield 100, providing the requisite EMI shielding for the coated regionsof the printed circuit board. As such, conductive coating 104 is appliedto the surface of dielectric coating 102 which has been appliedpreviously to selected regions of the printed circuit board. Due to thecomplete coverage provided by dielectric coating 102, conductive coating104 does not contact any portion of the printed circuit board regionthat has been coated previously by dielectric coating 102.

Generally, conductive coating 104 has the capability to adhere to thesurface of dielectric coating 102 so as to conformally coat and adhereto the underlying region of the printed circuit board. In certainembodiments, conductive coating 104 will also likely conformally coatand adhere to predetermined components of the printed circuit boarditself, particularly ground pads, strips and moats (collectively, groundlands) in the printed wiring board 202. Conductive coating 104 may alsobe required to adhere to other predetermined elements on the printedcircuit board in some applications. For example, in hybrid shieldingarrangements in which conformal EMI shield 100 is used in conjunctionwith a conventional metallic box, conformal EMI shield 100 preferablyadheres to a surface of such a metallic box.

As with dielectric coating 102, the relevant material properties ofconductive coating 104 include primarily viscosity and adhesion. Thecombination of these properties should be sufficient to enableconductive coating 104 to be applied via atomization spray techniquesand, once applied, to adhere to the surfaces in the immediate vicinityof where it was applied. Specifically and in one embodiment, theviscosity of conductive coating 104 can range from 10–40″ Zahn cup #3(full body). In another embodiment, conductive coating 104 has aviscosity of 15–30″ Zahn cup #3 (full body).

Conductive coating 104 has an adhesion suitable to enable it to adhereto the noted materials, in particular, dielectric coating 102. As theviscosity of conductive coating 104 decreases, the adhesiveness may needto increase to ensure conductive coating 104 adheres to the surface towhich it is applied in the immediate vicinity in which it is applied. Ingeneral, conductive coating 104 preferably has an adhesion thatsatisfies the ASTM 5B rating.

To supplement the adhesion of conductive coating 104 to dielectriccoating 102, in one embodiment dielectric coating 102 and conductivecoating 104 have the same or similar composite resin structures thatfacilitate bonding between the two coatings. Such a bonding will bemaintained over significant periods of time, preferably inclusive of thelife of the printed circuit board, due to the two coatings havingsimilar coefficients of thermal expansion. This reduces the shearingstresses between the two coatings as the printed circuit board and,hence, conformal EMI shield 100, heat and cool during the operationallife of the printed circuit board. For this reason, when dielectriccoating 102 includes the noted CQW-L200DF dielectric coating, it ispreferred that conductive coating 104 is the MQW-L85 conductive coatingnoted below due to the similarity of the composite resin structures.

In one embodiment, conductive coating 104 is an aqueous coatingcomposition with particles of conductive metal suspended therein. Suchconductive metals can be, for example, copper, silver, nickel, gold orany combination thereof. The ohmic resistance of conductive coating 104is between 0.05 and 0.2 ohms per square at a film thickness ofapproximately 1.0 mil. In one embodiment of conformal EMI shield 100,conductive coating 104 is TARA EMI-RFI shielding, Formula MQW-L85manufactured by The Egyptian Lacquer Manufacturing Company, Franklin,Tenn., USA. MQW-L85 is described in U.S. Pat. Nos. 5,696,196 and5,968,600 both of which are hereby incorporated by reference herein intheir entirety. MQW-L85 is designed for coating product enclosures orhousings such as those used in cellular phones. MQW-L85 has a viscosityin the range of approximately 15–20″ Zahn cup #3(full body).

The thickness of conductive coating 104 should be sufficient to preventthe passage of the electromagnetic radiation generated by the coatedprinted circuit board 304. It should be apparent that the thickness ofconductive coating 104 is a function of the type and characteristics ofthe materials used to form conductive coating 104. In one embodiment,conductive coating 104 is approximately 1.1±0.2 mils; that is, athickness in the range of 0.9 to 1.3 mils provides significant shieldingeffectiveness. However, it should be understood that in alternativeembodiments, conductive coating 104 has a thickness that depends on itsohmic resistance and desired shielding effectiveness at the anticipatedelectromagnetic frequencies to be shielded.

As with dielectric coating 102, MQW-L85 is preferably applied at roomtemperature, between 70–80 degrees Fahrenheit, although an applicationenvironment of 60–100 degrees Fahrenheit is suitable. Preferably,multiple cross-coats are applied for one or more layers of conductivecoating 104. After application, the MQW-L85 conductive coating 104 iscured at approximately 140–160 degrees Fahrenheit for approximately 30minutes. It should be understood that lower temperatures can be used,depending on the temperature tolerance of the printed circuit board. Thecuring time may need to be accordingly altered. However, it is preferredthat this embodiment of conductive coating 104 is cured at the notedtemperatures because the elevated temperature facilitate the alignmentof the metallic flakes. When the metallic flakes orient themselves inthis way, the conductivity of the conductive coating 104 is maximized.

A secondary effect of conductive coating 104 is that it is thermallyconductive. The heat generated by coated printed circuit board regionsare transferred through dielectric coating 102 to conductive coating 104which conducts through the surface of the board. The heat can thentravel off the printed circuit board, primarily by dissipating throughconvection or through conduction to a heat sink.

As with dielectric coating 102, conductive coating 104 can be applied tothe sharp edges and cavities of printed circuit board 304. This isillustrated in FIG. 2C in which conductive coating 104 covers dielectriccoating 102 on integrated circuit 204. Conductive coating 104 coats theside of integrated circuit body 206 behind leads 208, as well assubstantially all of the surface of leads 208 themselves. In thosecircumstances in which the gap between neighboring leads 208 is reduceddue to the presence of dielectric coating 102, conductive coating 104may bridge the gap as shown in FIG. 2C.

Although suitable for many applications of the present invention, thereare well-known environmental concerns associated with the use ofmetallized coatings that restrict and increase the cost of disposal,reuse, etc. Also, to avoid oxidation, an additional protective layer maybe applied to those portions of the printed circuit board that are notcoated with dielectric coating 102, namely, ground lands. Such awell-known protective layer prevents oxidation of the ground lands dueto oxidation.

To overcome these particular limitations of metallized coatings andbroaden the possible applications of conformal EMI shield 100, in oneaspect of the present invention, conductive coating 104 comprises anintrinsically conducting polymer (ICP). As the name implies,intrinsically conducting polymers are electrically conductive polymermaterials. Importantly, intrinsically conducting polymers are polymermaterials that have a significant conductivity without the addition, ordoping, of some other material such as a noble metal.

FIG. 12 is a scale illustrating the relative conductivity ofintrinsically conductive polymers, metal conductors, semi-conductors andinsulators. In this illustrative scale, conductivity is presented inunits of Siemens per centimeter (S/cm). The vertical scale in the middleof the figure sets forth the conductivity in increments of 100 S/cm,from 10⁻¹⁸ S/cm to 10⁸ S/cm. On the left-hand side of the scale arethree categories of well-known materials showing the range ofconductivity provided by each category of material. For example,insulators 1206 have a conductivity of approximately 10⁻⁸ to 10⁻¹⁸ S/cm.As evidenced by these values, insulators effectively inhibit electricalconduction. Examples of insulators include, for example, general-purposethermoplastics, polyethylene, polypropylene, PVC, polystyrene and PTFE.Semi-conductors 1204 have a conductivity of approximately 10⁻⁶ to 10⁰S/cm. Two well-known well-known semiconductor materials are germaniumand silicon. As shown in FIG. 12, the conductivity of metal conductors1202 is approximately 10² to 10⁶ S/cm. Examples of metals that servewell as conductors include, for example, copper, silver and gold. Thus,metal conductors have the greatest conductivity, semiconductorsgenerally have a lower conductivity and insulators allow for minimal orno conduction.

For ease of comparison, intrinsically conductive polymers are positionedon the right side of the scale. As shown, the conductivity ofintrinsically conductive polymers ranges from slightly greater than 10⁻⁸S/cm to slightly less than 10⁶S/cm, making them analogous to theconductive metals. Such conductivity levels make intrinsicallyconductive polymers suitable for use in conductive coating 104. Someexamples of intrinsically conductive polymers that can be used inaccordance with the teachings of the present invention include but arenot limited to polypyrrole, polyanaline, polyacetylene,polyththiophenes, poly(p-phenylele vinlene)s, poly-thylenedioxythiopheneand polyphenylenesulfide.

Once applied and cured, conductive coating 104 is a substantially dry,solid coating. However, the state of conductive coating 104 when priorto application can vary. Preferably, the conducting polymer is suspendedin a dispersion to facilitate the preferred application method ofconventional spray atomization techniques. Such a dispersion is referredto herein as a conductive polymeric dispersion. FIG. 13 is a series ofimages illustrating the state of one preferred embodiment of aconductive polymeric dispersion 1300 when applied (FIG. 13A), cured(FIG. 13B) and used (FIG. 13C) in accordance with the teachings of thepresent invention.

In FIG. 13A, conductive polymeric dispersion 1300 is shown after it hasbeen applied to dielectric coating 102 but before it is cured. In oneembodiment, conductive polymeric dispersion 1300 is a single componentpolymeric dispersion. Examples of such an embodiment include adispersion formed by suspending intrinsically conductive polymerparticles in water or an organic solvent. It is anticipated that incertain applications, such a dispersion may not cure to form aconductive coating 104 that uniformly covers and adheres well todielectric coating 102.

In contrast, the illustrative embodiment of conductive polymericdispersion 1300 illustrated in FIG. 13A is formed by suspending asubstrate resin particles 1302 in a base liquid 1304. As shown in FIG.13, substrate 1302 includes larger components, commonly referred to asbeads. Substrate 1302 is a material such as acrylic or polyurethane towhich intrinsically conductive polymer 1200 adheres. Thus, when combinedwith intrinsically conductive polymer 1200 in base liquid 1304,intrinsically conductive polymer 1200 coats the exterior surfaces ofsubstrate beads 1302 to form what is commonly referred to as acore-shell dispersion. Thus, the illustrative conductive polymericdispersion 1300 is referred to herein as a core-shell dispersion.

As one of ordinary skill in the art would find apparent, conductivepolymeric dispersion 1300 is a heterogeneous solution in which theintrinsically conductive polymer 1200 is dispersed in a base liquid 1304such as water or organic solvent. As the contiguity of the intrinsicallyconductive polymer component in the cured conductive coating 104increases, so too does the ability of conductive coating 104 to conductelectricity. Accordingly, it is preferable that the suspension issubstantially uniform to insure the contiguity of the curedintrinsically conductive polymer in the resulting conductive coating104.

With regard to base liquid 1304, waterborne dispersions are preferredbecause they are substantially easier to process that dispersions usingorganic solvents. In addition, the use of water eliminates theenvironmental and processing drawbacks associated with the use oforganic solvent emissions. However, organic solvents including, forexample, N-Methyl-Pyrolidinone (NMP), various alcohols, acetone,Methyl-Ethyl-Ketone (MEK), and others, may be suitable in certainapplications.

The state of conductive polymeric dispersion 1300 after curing is shownin FIG. 13B. In this aspect of the invention, conductive coating 104 isreferred to as a conductive polymeric coating. When cured, substratebeads 1302 bind together to form a contiguous, rigid surface to whichintrinsically conductive polymer 1200 adheres, forming conductivecoating 104. In addition, conductive polymeric dispersion 1300 includesother components 1306. Such other components can include materials tofacilitate a particular process or to alter a particular characteristic.To insure substrate beads 1302 adhere well to the printed circuit boardsurface, dispersion 1300 includes binder 1302. When cured, binder resinparticles 1308 bind to each other and solidify to form a relatively thinadhering layer over substrate beads 1302.

As shown in FIG. 13B, the liquid base 1304 in which the solids wereborne is removed during curing. Once removed, conductive coating 104includes substrate beads 1302 with their coating of conductive polymer1200 secured to dielectric coating 102 by a binder 1306. Binder 1306 isan acrylic or urethane that is suspended in dispersion 1300 (FIG. 13A)which becomes a solid when cured (FIG. 13B). Thus, when cured, substratebeads 1302 come together and contact each and are adhered to dielectriccoating 102 by binder 1306.

FIG. 13C illustrates the operation of conductive polymeric coating 104.When an electric charge 1310 is applied to conductive polymeric coating104, it travels across intrinsically conductive polymer 1200 coating theside of substrate beads 1302 opposite the side of substrate beads 1302in contact with dielectric coating 102.

As is well-known in the art, conductive polymeric dispersion 1300 caninclude components that facilitate a desired curing process. Forexample, in one embodiment, conductive polymeric dispersion 1300 can becured with ultraviolet (UV) light. In such an embodiment, othercomponents 1306 includes any of the well-known photosensitizing agentssuch as a UV-curable acrylic. Alternatively, certain embodiments of theconductive polymeric dispersion can be heat cured. In such embodiments,conductive polymeric dispersion 1300 includes a heat curing agent suchas an anhydride. In one preferred embodiment, additional materials areadded to conductive polymeric dispersion 1300 to facilitate a bothtemperature and UV curing. For example, in one embodiment, dispersion1300 includes the Shadowcure® material available from the LoctiteCorporation, Rocky Hill, Conn.

Shadowcure includes both photosensitizing and heat curing agents.Shadowcure enables dispersion 1300 to cure in response to both exposureto UV light and temperature. For similar reasons noted above, thisembodiment is desirable in those applications in which the printedcircuit board configuration is such that there are cavities betweencomponent leads, neighboring components and between components and thesurface of the printed wiring board. It should be understood by those ofordinary skill in the art that other additives can be included in theconductive polymeric dispersion to achieve a desired property orbehavior. For example, N-Methyl-Pyrolidinone (NMP) can be added to thedispersion to reduce the minimum film forming temperature.

The following properties of intrinsically conductive polymers thatimpart substantial benefits to conductive coating 104 includeconductivity, transparency and redox potential. With regard toconductivity, it was noted that intrinsically conductive polymers arehighly conductive, akin to noble metals. Conductivity increases as theapplied thickness of the conductive polymeric coating increases.Conductivity can also be modified by adjusting the concentration ofintrinsically conducting polymer 1200 in dispersion 1300. In oneembodiment, the conductivity of conductive coating 104 is betweenapproximately 10⁻⁸ to 10⁶ S/cm. In another embodiment, the conductivityis approximately between 10⁻² to 10⁶ S/cm. In a further embodiment, theconductivity is approximately between 10⁻¹ to 10⁶ S/cm. In a furtherembodiment, the conductivity is approximately between 10⁻¹ to 10² S/cm.It should be understood that the conductivity can be any of theconductivity values associated with each of the exemplary intrinsicconductive polymers illustrated in FIG. 12.

Another property of intrinsically conducting polymers is transparency.In certain circumstances, it may be desirable to be able to view theunderlying printed circuit board after it is coated with conformal EMIshield 100. This is more difficult with metallized coatings due to theiropaque. In contrast, polymeric conductive coating of this aspect of theinvention can be transparent. Intrinsically conductive polymers arerelatively transparent materials. Depending on the applied thickness ofconductive coating 104, the underlying components may be observable.

This transparency of conductive coating 104 can be modified by adjustingthe thickness of conductive coating 104. The thinner the layer ofconductive coating 104, the higher the transparency. Since thisrelationship is inverse to that of conductivity, a tradeoff between thetwo properties is to be selected to achieve a desired combination ofmaterial properties. The transparency of conductive coating 104 providesthe advantage of allowing for the viewing of the dielectric coating 102and printed circuit board components that are covered by conductivecoating 104.

Another property of intrinsically conductive polymers is that the areextremely corrosion resistant. This reduction in corrosion, oroxidation, is commonly referred to as redox potential. The redoxpotential of aluminum and iron, which are commonly found in conventionalmetallized coatings, is approximately −1.6 volts and −0.4 volts,respectively. This causes conventional metallized coatings to oxidizethe metallic surfaces to which they are applied. The uncontrolledmixture of iron oxides and—hydroxides corrodes the metal surfaces. Toprevent such an occurrence, a protective layer is often applied prior tothe metallized coating.

In contrast, the redox potential of conducting polymers is greater thanzero and, preferably, between 0 and 1 volt. In one embodiment, the redoxpotential is approximately +0.8 volts, about the same as the redoxpotential of noble metals such as silver. This causes the intrinsicallyconductive polymer to passivate the metallic surfaces to which they areapplied. That is, the redox potential of conducting polymers reduces theelectrochemical potential of the metal, causing the formation of a solidand strong protective iron oxide layer, for example, Fe₃O₄ or FeO(OH).This hard oxide layer protects the rest of the material againstcorrosive attack. Thus, a further advantage that makes this aspect ofthe present invention preferable is that corrosion on the metal parts ofthe printed circuit board is minimized or eliminated.

In one particular embodiment, the conductive polymeric dispersion is theConQuest® conductive dispersion available from DSM Research, TheNetherlands. (ConQuest® is a registered trademark of DSM N.V. CompanyNetherlands.) For example, in one particular application, the ConQuestXP 1000 family of waterborne dispersions is preferred. ConQuest XP 1000is a water-borne, electrically conductive dispersion that derives itselectrical conductivity from the electrically conductive polymerpolypyrrole. This product family as approximately a 20% solids content,and can be further diluted with the addition of water or with isopropalalcohol or another waterborne dispersion. Other properties include a pHof 2–4 and a coating conductivity of >0.2 S/cm.

ConQuest XP 1000 can be applied to various substrates found in printedcircuit boards. Improved adhesion can be obtained by adding otherstrongly adhesive, waterborne dispersions. Addition levels up to 30%(based on solids) of non-conductive compatible dispersions will notaffect the conductivity to a great extent. Examples of compatibledispersions include Uradil AZ554 Z-50, VV 240 SC 341 and SC 162 DSMResins, all of which are available from DSM Research.

ConQuest XP 1000 has a Minimum Film Forming Temperature (MFT) of 50° C.Regular drying temperatures are between 60° C. and 120° C. Thistemperature can be reduced by adding NMP (N-Methyl-Pyrrolidinone) orDPNB (Di-PropyleneglycolN-Buthylether). Room temperature film formingproperties are achieved at a 10% (DPNB) or 25% (NMP) addition level,based on the amount of solids in the formulation.

To achieve a desired conductivity, the ConQuest XP 1000 conductivepolymeric dispersion is applied in one or more applications to a drylayer thickness of approximately 3 microns depending on theconcentration of the conducting polymer in the conductive polymericdispersion. In an alternative embodiment, the applied thickness isapproximately 3–5 microns.

It should be appreciated that there are numerous variations of the aboveembodiments that can be altered while remaining within the scope of thepresent invention. For example, in certain embodiments, a novalac-typeprocess is used.

One advantage of this latter embodiment of conductive coating 104 overconventional techniques is that the ICPs in the conductive coating donot suffer from the well known environmental concerns associated withmetals. In particular, the ICPs in conductive coating 104 can be easilyand cost effectively disposed. In addition, the material can be recycledeasily, increasing manufacturing yield. This advantage becomes even moresignificant when the conductive polymeric dispersion utilizes a baseliquid of water. Such waterborne dispersions eliminate the environmentalproblems associated with organic solvents.

Another advantage of the present invention is that the ICPs aretransparent. Depending on the applied thickness, the underlyingcomponents can be observed. This is in contrast to current metal coatingwhich are opaque. A further advantage of the invention is that ICPs areresistant to corrosion. Unlike metallized coatings, there is no need toapply an additional protective layer to prevent oxidation.

3. A Printed Circuit Board With a Conformal EMI Shield

-   -   A. General

FIG. 3 is a cross-sectional view of a portion of a printed circuit board304 with conformal EMI shield 100 of the present invention appliedthereto to cover the exposed surfaces of selected portions of printedcircuit board 304. Printed circuit board 304 of the present inventioncomprises, generally, printed wiring board 202 with components 302mounted thereon, with both shielded at least in part, and preferablycompletely, with conformal EMI shield 100 of the present invention. Inthe embodiment illustrated in FIG. 3, conventional metal cages 316A and316B are utilized to shield connector wires 320A and 320B of I/Oconnector 318. Conformal EMI shield 100 is applied to desired regions orportions of printed circuit board 304. Such regions or portions includeregions of printed wiring board 202 as well as all or part of certaincomponents 302.

Printed circuit board 304 includes a memory card 306 mounted on printedwiring board 202. Memory card 306 is shielded by a conventional metalcage 316B. Printed circuit board 304 also includes integrated circuit204 introduced above with reference to FIGS. 2A–2C, a resistor 310 and apower feed-through connector 308. Power feed-through connector 310carries low frequency signals and, therefore, does not need to beshielded. In contrast, another type of connector mounted on printedwiring board 202 is shielded connector 312. Connector 312 receives, forexample, high speed data signals. Shielded connector 312 has an EMIshield (described in detail below) whereas power feed-through connector308 does not.

A metal cage 316A shields I/O cables or leads 320A and 320B of I/Oconnector 318. I/O connector 318 may be, for example, an RS232connector, among others. Metal cage 316A includes a surface-mountedfeed-through capacitor 314 for preventing signals from being conductedout of metal cage 316A on the low frequency signal traces to which it isconnected. Capacitor 314 has a lead in the form of solder spots and isconnected to a ground connection.

-   -   B. Printed Circuit Board Coverage

In accordance with one preferred embodiment, coated printed circuitboard 304 is completely shielded with conformal EMI shield 100. That is,conformal EMI shield 100 is a continuous coating covering all surfacesof printed circuit board 304. However, conformal EMI shield 100 need notcover the entire printed circuit board 304. For example, in oneembodiment, there may be regions of printed circuit board for which EMIprotection is unnecessary. In other circumstances, such as that shown inFIG. 3, other shielding mechanisms can be implemented on printed circuitboard 304 in combination with conformal EMI shield 100 to provide therequisite EMI shielding.

In FIG. 3 metal cages 316 are used to shield I/O connector 318 leads 320and memory card 306. In addition, ancillary parts of a product whichgenerate minimal or no electromagnetic radiation do not warrantprotective measures to be employed to limit such emissions. Such devicesinclude, for example, interconnecting cables, power supplies, diskdrives, etc. These and other, similar devices do not need to be coatedwith the conformal EMI shield of the present invention. As a result,access to such components and subassemblies can be made simpler. Thus, aprinted circuit board 304 of the present invention is one that is atleast partially coated with one embodiment of conformal EMI shield 100.In FIG. 3, this conformal EMI shield 100 covers a portion of top surface322 of printed circuit board 304 in which components 302 are mounted, aswell as a bottom surface 326 of printed circuit board 304.

As one of ordinary skill in the art would find apparent, differenttechniques can be implemented to apply conformal EMI shield 100 tospecific regions of printed circuit board 304. For example, in oneembodiment, conformal EMI shield 100 is selectively applied to thedesired portions of a printed wiring board or components mounted thereonusing highly directional air spraying techniques. Alternatively, printedcircuit board 304 is masked before application of the dielectric coating102 to avoid application to those regions of printed circuit board 304that are not to be shielded.

-   -   C. Grounding of Conformal EMI Shield

Conductive coating 104 is preferably grounded at various locations onprinted circuit board 304. In the following embodiments, conformal EMIshield 100 is connected electrically to a ground plane in printed wiringboard 202. Two embodiments of making such a ground connection areillustrated in FIGS. 4 and 5. FIG. 4 is a cross-sectional view of aground moat surrounding shielded connector 312 illustrated in FIG. 3.FIG. 5 is a cross-sectional view of a ground pad mounted on printedwiring board 202.

Referring now to FIG. 4, conformal EMI shield 100 is preferably groundedthrough a ground moat at locations where wires, leads, cables, etc.,carrying high frequency signals are connected to printed wiring board202. Conformal EMI shield 100 effectively provides a conductive looparound the signal wire connected to shielded connector 312. A currentcan be induced in that portion of conductive coating 104 surroundingshielded connector 312 due to the transmission of high frequency signalsthrough the connector. To prevent such a current from traveling to otherportions of printed circuit board 304 or to emanate off of the surfaceof conductive coating 104, ground moat 402 is provided in printedcircuit board 304 surrounding signal connector 312. To insure completeshielding, ground moat 402 preferably surrounds completely shieldedconnector 312. One or more vias 406 connect ground moat 402 to groundplane 404. In the embodiment shown in FIG. 4, the vias 406 are blindvias since they do not pass through to the other side of printed wiringboard 202. Preferably, there are a number of vias 406 distributed aroundground moat 402 to reduce the distance of the conductive path to groundplane 404. Any signals generated in conductive coating 104 areimmediately shunted to ground plane 404 through ground moat 402 and avia 406.

Note that dielectric coating 102 is applied to printed wiring board soas to not cover the surface of ground moat 402 and shielded connector312. In one embodiment, this is achieved by masking ground moat 402 andshielded connector 312 prior to applying dielectric coating 102.Conductive coating 104 is applied so as to coat dielectric coating 102as well as ground moat 402. This is achieved by removing the mask fromground moat 402 and masking shielded connector 312 prior to applyingconductive coating 104. Importantly, either ground moat 402 and/orconductive coating 104 are electrically connected to shield 408 ofshielded connector 312. Thus, any interference generated along thelength of the signal lead, connector or conductive coating 104 isimmediately shunted to ground. Thus, a ground moat 402 mounted onprinted wiring board 202 completely around shielded connector 320 andconnected electrically to a shield 408 of connector 312 and a groundplane 404 eliminates the EMI that can be transmitted by conductivecoating 104 in the vicinity of shielded connector 312.

FIG. 5 is a cross-sectional view of a ground pad 502 mounted on printedwiring board 202. In one embodiment, conformal EMI shield 100 isgrounded periodically through such ground pads 502 across all regions ofconformal EMI shield 100. In certain applications, the performance ofconformal EMI shield 100 is improved when it is grounded periodically.In one embodiment, this is achieved by providing one or more ground pads502 across the shielded regions of printed circuit board 304. One suchground pad 502 is illustrated in FIG. 5, although there are many otherembodiments which can be implemented.

Ground pad 502 is a surface-mounted conductive pad connected to groundplane 404 through blind via 406. As with the embodiment illustrated inFIG. 4, dielectric coating 102 is applied to printed wiring board 202 soas to coat the surface of printed wiring board 202 and not ground pad502. Conductive coating 104 is applied so as to coat dielectric coating102 and ground pad 502. This connects electrically conductive coating104 to ground plane 404.

In an alternative embodiment, vias 406 transect the entire printedwiring board 202; that is, they extend from ground plane 404 to bothsurfaces of printed wiring board 202. Accordingly, one embodiment ofprinted circuit board 304 is preferably arranged to take advantage ofsuch ground vias. For example, shielded connectors 312 and correspondingground moats 402 can be mounted on opposing sides of printed wiringboard 202. Alternatively, ground pads 502 or a combination of groundmoats 402, ground pads 502, ground strips or other combinations ofground lands can be disposed on opposing sides of printed wiring board202.

It should be understood that the location, quantity and distribution ofground lands in general, and ground moats 402 and ground pads 502specifically, can vary significantly depending on a number of well-knownfactors and features of printed circuit board 304. For example, thequantity of signal leads that come onto or off of printed wiring board202, the frequency of the signals traveling on the signal leads. Inaddition, the resulting electromagnetic fields that are generated by thesignals, which is based on the type of lead and connector as well as thesignal characteristics will also determine the grounding schemeimplemented. Referring to FIG. 3, for example, ground moat 402 may belocated at various locations on printed circuit board 304, depending onthe type of signals and components implemented. For example, ground moat402 can be mounted on bottom surface 326 of printed wiring board 202around the location at which I/O leads 320 enter printed wiring board202.

-   -   D. Electrically Connecting EMI Shielded Regions

As noted, conformal EMI shield 100 can be applied to predeterminedregions or portions of printed circuit board 304. Referring to theexemplary embodiment illustrated in FIG. 3, conformal EMI shield 100coats top surface 322 of printed circuit board 304. This coating isphysically contiguous, surrounding such elements as metal cages 316,shielded connector 312 and power feed-through connector 308. Similarly,in the embodiment disclosed in FIG. 3, conformal EMI shield 100 alsocoats entirely bottom surface 326 of printed wiring board 202.

A potential can develop between the region(s) of conformal EMI shield100 that coat top surface 322 and the region(s) of conformal EMI shield100 that coat bottom surface 326. Should such a potential develop, thetwo regions of conformal EMI shield 100 can effectively form an RFantenna and, therefore, be a source of EMI. To prevent this fromoccurring, the top and bottom surface regions of conformal EMI shield100 are preferably connected electrically to each other, directly orthrough a common ground. Thus, conformal EMI shield 100 is anelectrically continuous coating that may or may not be physicallycontiguous over the surfaces of printed circuit board 304.

In the embodiment illustrated in FIG. 6A, the two regions of conformalEMI shield 100 that coat the top and bottom surfaces of printed circuitboard 202 are connected to each other through another region ofconformal EMI shield 100 applied to edge surfaces 324 of printed wiringboard 202. In other words, the three regions (top, edge and bottomcoatings) can be considered a single region and printed circuit board304 is coated continuously on the top, edges and bottom surfaces withconformal EMI shield 100. Thus, conformal EMI shield 100 is, in thisembodiment, physically contiguous and electrically continuous.

Should it be impracticable or otherwise undesirable to apply conformalEMI shield 100 to edges 324 of board 202, then alternative arrangementscan be implemented to provide electrical continuity between all regionsof conformal EMI shield 100. For example, in an alternative embodimentshown in FIG. 6B, printed wiring board 202 can be made with platededges. Edges 324 of board 202 are preferably plated with the samematerial as the material utilized in ground plane 404, such as copper.The top and bottom regions of conformal EMI shield 100 are connected toedge plating 604 on each side of board 202. As shown in FIG. 6B, edgeplating 604 wraps around printed wiring board 202 to form ground strips601 which cover some distance or area on top and bottom surfaces 322,326 thereof. As used herein, a ground strip 601 is an elongate groundpad.

Ground strips 601, edge plating 604 and ground plane 404 are connectedphysically and electrically. Dielectric coating 102 is applied toprinted wiring board 202 so as to coat top and bottom surfaces 322, 326of printed wiring board 202 and not ground strips 601. Conductivecoating 104 is applied so as to coat dielectric coating 102 and at leasta portion of ground strips 601, as shown in FIG. 6B. This provides anelectrical connection between conductive coating 104 on the top andbottom surfaces 322, 326 to each other as well as to ground. Thus, inthis alternative embodiment, conformal EMI shield 100 includesphysically separate regions that are connected electrically through edgeplating 604.

FIG. 6C is a cross-sectional view of an alternative approach toachieving electrical continuity between regions of conformal EMI shield100. In this alternative embodiment, printed wiring board 202 ismanufactured with rows of ground vias 606 and one or more ground strips601 around its periphery. As noted, a ground strip 601 is an elongateground pad. On each side 322, 326 of printed wiring board 202, the vias606 are connected electrically to ground strips 601. As in theembodiment illustrated in FIG. 6B, dielectric coating 102 is applied toprinted wiring board 202 so as to coat top and bottom surfaces 322, 326of printed wiring board 202 while not coating ground strips 601.Conductive coating 104 is applied so as to coat dielectric coating 102and at least a portion of ground strips 601. This connects electricallyconductive coating 104 on the top and bottom surfaces 322, 326 to eachother as well as to ground.

FIGS. 6D and 6E are cross-sectional views of an edge region of printedwiring board 202 showing different embodiments for connectingelectrically regions of conformal EMI shield 100 that coat the top andbottom surfaces 322, 326 of printed wiring board 202 using spring clips.Specifically, in FIG. 6D ground strips 601 are mounted on top and bottomsurfaces 322, 326 of printed wiring board 202 proximate to edge surfaces324. One or more spring clips 602 are secured around edge 324 of printedwiring board 202 so as to contact ground strips 601 secured to opposingsides of printed wiring board 202. Sprint clip 602 is formed from anelectrically conductive material, and is preferably a unitary devicethat can be installed manually. As with the other embodiments,dielectric coating 102 is applied to printed wiring board 202 so as tocoat top and bottom surfaces 322, 326 of printed wiring board 202 whilenot coating ground strips 601. Conductive coating 104 is applied so asto coat dielectric coating 102 and at least a portion of ground strips601. This connects electrically conductive coating 104 on the top andbottom surfaces 322, 326 to each other through spring clip 602. Itshould become apparent that each ground strips 601 has a size or lengthsufficient to enable spring clip 602 to attach securely thereto, withoutrisk of inadvertent detachment.

In the embodiment illustrated in FIG. 6E, conformal EMI shield 100 coatsthe entire top and bottom surfaces 322, 326 in the vicinity proximate toedge surface 324. In such embodiments, ground strips 601 shown in FIG.6D are eliminated, with spring clip 602 contacting directly conductivecoating 104. As one of ordinary skill in the art would find apparent,other configurations may be implemented to connect electrically regionsof conformal EMI shield 100 coating top and bottom surfaces 322, 326 ofprinted circuit board 304.

-   -   E. Design of Printed Wiring Board to Accommodate EMI Shield

Aspects of the present invention include a printed wiring board 202constructed and arranged to operate with conformal EMI shield 100, aswell as a printed circuit board 304 incorporating such a printed wiringboard 202 and conformal EMI shield 100.

Printed wiring board 202 typically includes multiple layers each ofwhich includes an insulator, commonly an epoxy glass, with signal tracesand a ground plane formed on opposing surfaces thereon. Typically,traces internal to printed wiring board 202 are located between twoground planes with an intervening layer of insulating material. Signaltraces that travel long the surface of the printed wiring board arepositioned between a ground plane below, with an intervening layer ofinsulating material, and air above.

The characteristic impedance of the signal traces is a function of thewidth and thickness of the trace, the distance between the trace andsurrounding ground plane(s), and the dielectric properties of theintervening insulating material. The characteristic impedance in turneffects the electrical properties of the traces such as the velocity ofpropagation.

The greatest contributor to the characteristic impedance of a signaltrace is the parasitic capacitance established between the signal traceand its neighboring traces. Since internal traces have a ground planelocated above and below it while a surface trace has a single groundplane located below it, the parasitic capacitance of the internal traceis approximately twice that of surface traces, with a concomitantreduction in characteristic impedance.

This is not the case for printed circuit boards of the presentinvention. Coating a printed wiring board 202 with conformal EMI shield100 will significantly increase the parasitic capacitance of the surfacetraces, decreasing the characteristic impedance of the surface traces.The change in the characteristic impedance is, as noted, a function ofthe cross-section of the surface trace, the distance between the surfacetrace and conductive coating 104 and the dielectric properties ofdielectric coating 102.

Thus, in accordance with aspects of the present invention, printedwiring board 202 and conformal EMI shield 100 are configured to controlelectrical characteristics of surface traces by adjusting such features.For example, the width and thickness of the surface traces as well asthe dielectric constant and thickness of dielectric coating 104 can beadjusted to achieve desired electrical characteristics such ascharacteristic impedance. In an alternative embodiment, printed circuitboard 102 can be configured with no traces on the outer board layers.

In addition, a printed wiring board 202 of the present inventionincludes ground moats 402 mounted around connectors that may carry highfrequency signals, as described above, and, preferably, ground landsperiodically mounted throughout printed wiring board 202.

4. Individual Components Coated with Conformal EMI Shield

Repair of printed circuit boards 304 coated with conformal EMI shield100 is likely to be difficult and expensive. The ideal solution would beto coat mainly the inexpensive parts of printed circuit board 304, suchthat it would be economical to merely discard failed or defectiveboards, salvaging the expensive processors, etc. for reuse. However,such components would lack the appropriate shielding. Aspects of thepresent invention provide a technique for coating such components withconformal EMI shield 100 while enabling the components or subassembliesto be removable for repair, replacement or salvage.

FIG. 7 is a cross-sectional view of an exemplary embodiment of aremovable component, memory card 306, coated with conformal EMI shield100. In this exemplary embodiment, a workstation or desk top computerprovides the capability to be configured more or less memory as neededfor the computer's particular application. To accomplish this, a printedcircuit board with memory sockets to receive various combinations ofmemory cards is included in the device. Such memory cards can be pluggedinto the socket and shielded with conformal EMI shield 100 with theother components 302 on printed circuit board 304. Alternatively, such amemory card can be shielded with a conventional metal cage 316. As shownin FIG. 3, when such a conventional metal cage is utilized, the cage isconnected to conformal EMI shield 100 through, for example, gaskets orflanges that are bonded to conformal EMI shield 100.

In accordance with another aspect of the invention, memory card 306 canbe coated individually; that is, conformal EMI shield 100 can be appliedto memory card 306 prior to it being installed in printed circuit board304. Embodiments of such aspects of the invention include a mechanism toelectrically connect conformal EMI shield 100 coating memory card 306with conformal EMI shield 100 coating printed wiring board 202. In theembodiments shown in FIG. 7, such an electrical connection is achievedthrough the use of mating shielded connectors 702 and 312. As shown,connector 312 is physically and electrically connected to conformal EMIshield 100 applied to printed wiring board 202.

Most computers need to accommodate accessory cards from various vendorsthat add special capabilities. Examples include cards that provide theinterface to a particular LAN protocol, or a high-speed data interface.In one embodiment, the devices have special features to interface withconformal EMI shield 100. In a more preferred embodiment, the devicesare individually coated with conformal EMI shield 100, as describedabove. In further embodiments, a local shielding enclosure 316 such as ametal enclosure with appropriate removable covers for installation ofthe accessory cards can be used. The interface between the shielding ofthe metal enclosure and the coated board would be as described above inconnection with hybrid shielding arrangements, such as gaskets betweenthe enclosure and ground strips 603 on printed wiring board 202.

5. A Low Profile Component Cover for Encasing Components

In certain aspects, the invention includes a pre-manufacturednon-electrically conductive component cover. Generally, the componentcover is configured for placement over a printed circuit board componentand secured to the printed wiring board. The component cover and printedwiring board surround the component, forming an enclosure referred to asa component compartment. The component cover has a substantially thincross-section and an interior surface that follows closely the surfaceof the component, thereby minimizing the volume enclosed by thecomponent cover. In addition, the interior surface of the componentcover is immediately adjacent to the component so as not to addsignificantly to the dimensions of the printed circuit board. As such,the component cover has a low profile and prevents the subsequentlyapplied conformal EMI shield from physically contacting the encasedcomponent. Instead, the exterior surface of the component cover iscoated with the EMI shield. This provides the significant benefits ofthe conformal EMI shield while providing access to the compartmentalizedcomponent. This enables the covered component to be removed from theprinted circuit board for repair, replacement or salvage without havingto risk damage to the printed wiring board or component that may occurwith the removal of a conformal EMI shield applied directly to thecomponent.

Specifically, it may be required or desired to access certain components302 mounted on printed wiring board 202. For example, during theoperational life of printed circuit board 304, it may be desired toaccess a component 302 for troubleshooting, repair or replacement. Inaddition, it may be desired to salvage a component 302 at the end of theoperational life of printed circuit board 304. Such components 302 mayinclude, for example, expensive or rare components.

As noted, conformal EMI shield 100 completely coats those surfaces towhich it is applied. Removal from printed wiring board 202 of acomponent 302 coated with conformal EMI shield 100 requires that shield100 be severed at those locations where the component is connected oradjacent to printed wiring board 202. For example, referring to FIG. 2C,this may include the boundaries between printed wiring board 202 andintegrated circuit body 206 and leads 208.

There are a number of currently available techniques that could be usedto sever conformal EMI shield 100. One such conventional approach is tochemically etch or otherwise dissolve conformal EMI shield 100.Unfortunately, such treatments typically include the use of chemicalsthat are sufficiently active not only to penetrate conformal EMI shield100, but to also damage the coated components 302. In addition, theaccuracy of the application is limited, making it difficult to preciselyapply the chemicals to remove specific areas of conformal EMI shield100. As a result, severing conformal EMI shield 100 at component-boardboundaries around, for example, component leads, would be inefficient.

Another conventional technique that could be used to sever conformal EMIshield 100 is referred to as sandblasting or, more particularly, as beadblasting. However, such an approach also lacks precision and risksdamage to the coated component 302, particularly fragile components.Furthermore, even if components 302 can be successfully removed fromprinted wiring board 202, all surfaces of component 302 including itsbody and leads, will be coated with conformal EMI shield 100, as notedabove. This may interfere with the intended activities or future use ofthe component.

There are two options currently available to avoid such drawbacks oftraditional approaches. One approach is to not coat fragile andexpensive components 302 with conformal EMI shield 100, in which casethe component would not be shielded. An alternative approach is tocontain the component 302 within a conventional metallic cage 316, inwhich case it will suffer from the drawbacks noted above. Aspects of thepresent invention described below overcome the above and other drawbacksof chemical etching and bead blasting while not preventing the use andattendant benefits of conformal EMI shield 100.

In one aspect of the invention, a pre-manufactured,non-electrically-conductive, low profile component cover is secured toprinted wiring board 202, forming a sealed compartment dimensioned toencase component 302. Conformal EMI shield 100 can then be applied tothe exterior surface of the component cover in the manner describedabove. Since the component cover has a low profile, the coveredcomponent 302 experiences the same benefits of conformal EMI shield 100as if covered directly with conformal EMI shield 100. Here, however,conformal EMI shield 100 will not interfere with future uses of thecovered component. At least a portion of the cover, along with conformalEMI shield 100 attached thereto, can be easily removed from printedcircuit board 304 to expose component 302. Component 302 is thereafteraccessible, and can be tested or removed from printed wiring board 202using conventional techniques. In sum, components enclosed in acomponent compartment of this aspect of the invention are accessiblewhile enjoying the many advantages of conformal EMI shield 100.

FIG. 8A is a cross-sectional view of one embodiment of a component 302disposed in a sealed component compartment 804A formed by placing anon-electrically-conductive, low profile component cover 802A overcomponent 302, and securing component cover 802A to printed wiring board202. Component cover 802A in FIG. 8A has a surface of rotation about avertical axis 828, defining, in this embodiment, a symmetricalhalf-sphere. In an alternative embodiment shown in FIGS. 8B–8D a morearbitrarily shaped component cover 802B is shown. There, component cover802B forms with printed wiring board 202 an arbitrarily-shaped componentcompartment 804B for a processor integrated circuit 850.

In the embodiment illustrated in FIG. 8A, nonconductive component cover802A is preferably a pre-manufactured cover with a dome 822 configuredto envelop a selected component 302, and a flange 812 configured to besecured to printed wiring board 202. Dome 822 has a closed top 806, anopen bottom 810 remote from top 806, and walls 808 extending betweenclosed top 806 and open bottom 810, forming a recess 818 suitable forreceiving component 302. Flange 812 surrounds open bottom of dome 822and has a generally planar bottom surface 814 to mate with printedwiring board 202. When attached to printed wiring board 202, componentcover 802A and printed wiring board 202 form component compartment 804A.Component cover 802A can be unitary, or dome 822 and flange 812 areseparately manufactured pieces that are attached to each other to firman integral cover. Dome 822 and flange 812 can be detachably orpermanently connected using an appropriate non-electrically-conductiveadhesive.

Component cover 802A is sealed to printed wiring board 202. Preferably,the junction between component cover 802A and printed wiring board 202are sealed so as to prevent dielectric coating 102 from penetratingcomponent compartment 804A. Preferably, component compartment 804A isevacuated and sealed to remove moisture from compartment 804A andprevent corrosion of component 302. Any commonly known technique can beused to create a vacuum in compartment 804A. For example, the sametechnique as that commonly used to mount an integrated circuit can on aprinted wiring board can be used.

As noted, one important feature of component cover 802A is that it notprevent access to covered component 302. In one embodiment, componentcover 802A is sufficiently thin and formed from a material that can bemanually cut. In an alternative embodiment illustrated in FIG. 8A, aline of severability 816 traverses component cover 802A at the boundarybetween dome 822 and flange 812. Preferably, line of severability 816 isa line of weakening that facilitates the severing of dome 822 fromflange 812, leaving flange 812 secured to printed wiring board 202. Inone embodiment, line of severability 812 is a crease, fold line or otherweakened form. FIG. 8E shows two embodiments of a crease line 824. InFIG. 8E-1, crease 824A is v-shaped groove pointing towards the interiorcorner formed by flange 812 and wall 808. In FIG. 8E-2, crease 824B islaterally directed across wall 808. Such embodiments substantiallyreduce the thickness of component cover 802A at that location,facilitating severing of the portion of the cover traversed by the lineof severability, here, dome 822. Such severing may be achieved byscoring conformal EMI shield 100 (not shown) and cover 802A. In certainembodiments, the material, wall thickness and depth of crease 824 may besufficient to enable a technician to score conformal EMI shield 100 andsever dome 822 manually.

It should be understood that the location and type of line ofseverability 816 can be selected for a given application. For example,the noted embodiments of line of severability 816 do not provide anopening into compartment 804A. Such embodiments enable compartment 804Ato be evacuated, as noted above. However, should component 302 not besubject to corrosion or otherwise benefit from such an evacuation, thenline of severability 816 could be implemented as a line of perforationsor other embodiment which partially penetrates the walls 808 of dome822.

Returning to FIG. 8A, in an alternative embodiment, dome 822 ofcomponent cover 802A is pressure-rupturable. When walls 808 aresubjected to a manual force applied radially inward, dome 822 rupturesand is severed along line of severability 816. In such embodiments, theinterior surface 820 of dome 822 would not touch the component 302 asshown in FIG. 8A; rather, a space sufficient to enable the ruptured dome822 to separate from flange 812 would be provided. Thus, in such anembodiment, to expose component 302, conformal EMI shield 100 is cut atthe junction between dome 822 and flange 812. In those embodiments inwhich line of severability is a crease, the crease can guide the pointof a knife or other cutting instrument. Manual force is then applied towalls 808 adjacent to flange 812, severing dome 822 from flange 812.Dome 822 is thereafter removed to expose component 302.

Preferably, recess 818 is dimensioned to receive component 302 withminimal space between the interior surface 820 of dome 822 and component302 when component cover 802A is secured to printed wiring board 202.This, in conjunction with the relatively thin top 806, walls 808 andflange 812, results in a component compartment 804A having a minimalprofile. In other words, the volume of compartment 804A in notsubstantially greater than the volume defined by the surfaces ofcomponent 302.

An important feature of component cover 802A is that it have a shapesuitable for receiving dielectric coating 102 and, ultimately,conductive coating 104, while providing this minimal profile. As such,the exterior surface 826 of component cover 802A is preferably withoutsharp edges, indentations, or other abrupt changes. Thus, dome 822 cantake on virtually any shape beyond the symmetrical half-sphere shapeillustrated in FIG. 8A. For example, dome 822 can be disk-shaped,elliptical, rectangular and the like.

In another embodiment illustrated in FIGS. 8B-8D, a component cover 802Bhas a contoured, arbitrary shape. FIG. 8B is a cross-sectional view ofcomponent cover 802B dimensioned to cover a processor IC 850. FIG. 8C isa cross-sectional view of component cover 802B with a dielectric coating102 covering the exterior surface thereof, while FIG. 8D is a same viewshowing a conductive coating 104 covering dielectric coating 102 formingconformal EMI shield 100 of the present invention.

Referring to FIG. 8B, there is no distinctive boundary between dome 822Band flange 812B due to the contoured shape. A line of severability (notshown) can be formed in component cover 802B at any location above whereflange 812B is attached to printed wiring board 202.

Referring to FIG. 8C, dielectric coating 102 is applied to the surfaceof printed wiring board 202 and exterior surface 826B of nonconductiveconformal cover 802B. Similarly, as shown in FIG. 8D, conductive coating104 is applied so as to cover entirely dielectric coating 104 appliedpreviously to cover 802B.

Component cover 802 is, as noted, pre-manufactured with dimensionssuitable for covering completely a particular component 302. Componentcover 802 can be formed, folded or molded using any well-known techniquesuitable for the material used and intended application. With regard tomaterials, component cover 802 can be manufactured using any combinationof non-conductive materials. For example, component cover 802 can beformed of polyethylene terephthalate (PETE), polyphenylsulfone (PPS) orRTV silicone rubbers, and polymers and synthetic rubbers such as TEFLONand VITON, among others. (TEFLON and VITON are registered trademarks ofE. I. Du Pont de Nemours and Company.)

In alternative embodiments, component cover 802 is configured to provideaccess to component 302 without severing component cover 802. Forexample, in one alternative embodiment, component cover 802 is formedwith an aperture at top 806 and includes in combination a cover, beveledinsert or the like that can be removably inserted into the aperture. Togain access to component 302, conformal EMI shield 100 around thebeveled insert is scored and the insert removed. When component cover802 is to be subsequently shielded, the beveled insert is reintroducedinto the aperture and conformal EMI shield 100 is reapplied to componentcover 802.

Thus, the low profile, non-electrically conductive component covers 802enable components 302 to be shielded with conformal EMI shield 100located at a location immediately adjacent to the components, in thenear or induction field. In addition, component cover 802 does notprovide any EMI shielding function, enabling a myriad of materials andmanufacturing techniques to be used make such covers.

FIG. 11 is a flow chart of the primary operations performed in utilizinga component cover shown in FIGS. 8A–8E with conformal EMI shield of thepresent invention. At block 1102 the exterior dimensions of thecomponent is determined. This includes all features of the component,including leads, heat sinks, etc. this information is used to determinethe shape and size of dome 822 of component cover 802. Similarly, todetermine the appropriate dimensions of flange 812, the space around thecomponent is measured at block 1104. From this measurement, the size andshape of flange 812, including the configuration of bottom surface 814are determined.

At block 1106 the component cover 802 is manufactured based on thedimensions determined at blocks 1102 and 1104. Alternatively, component302 and component cover 802 can be specified prior to the manufacturingof printed wiring board 202. In such embodiments, printed wiring board202 is manufactured to accommodate flange 812 of component cover 802.

The component compartment 804 is formed at block 1108. Here, componentcover 802 is attached to printed wiring board 202 to form componentcompartment 804 dimensioned to encase component 302. Componentcompartment 804 is preferably evacuated or filled with a suitable inertatmosphere and sealed to maintain the environment at least untildielectric coating 102 is applied to component cover 802.

Conformal EMI shield 100 is applied at blocks 1110 and 1112. At block1110, dielectric coating 102 is applied to printed wiring board 202 andthe exterior surface of component cover 802. The manner in whichdielectric coating 102 is applied is described elsewhere herein. Asnoted, dielectric coating 102 can be applied in many layers each bondedwith its neighboring layers to form dielectric coating 102. At block1112, conductive coating 104 is applied to the surface of dielectriccoating 102. Each step 1110 and 1112 includes a number of subsidiarysteps to prepare the surface, cure the coating, etc. This is describedin greater detail above. Thus, upon the completion of the operationsnoted in block 1112, a conformal EMI shield 100 is applied to thecomponent 302 contained in the component compartment. Since thecomponent compartment is constructed and arranged to have a low profile,it defines a volume not substantially different than the volume definedby the surface of the covered component. As a result, conformal EMIshield 100 remains in the induction region immediately adjacent tocomponent 302.

6. Filler Material for use With Board-Level Containment ofElectromagnetic Emissions

Aspects of the conformal EMI shield of the present invention can includea high viscosity, non-electrically-conductive filler material forapplication to printed circuit board regions that have surfaces that arecavitatious and/or which have sharp edges or other highly variablesurface tangent slopes. The filler material and associated methodologiesof the present invention are preferably used in conjunction withconformal EMI shield 100. The high viscosity, electricallynon-conductive filler material substantially covers, and preferablyinfills, each cavity such that the covered cavity is thereafterinaccessible. The filler material also coats the sharp edges on theprinted circuit board. Thus, the pretreated portions of the printedcircuit board regions have a contiguous, contoured surface thatfacilitates the coating of the printed circuit board regions withconformal EMI shield 100.

Specifically, there are small gaps or spaces between component leads,neighboring components and between components 302 and printed wiringboard 202 that are relatively small. These various spaces are referredto herein generally and collectively as “cavities.” Such cavities mayhave more than one opening on the surface of the printed wiring boardthat exposes the cavity. For example, the space between the leads of acomponent and the component body and printed wiring board is consideredto be a cavity. Such a cavity has an opening to the surface of theprinted circuit board between neighboring leads. Significantly,dielectric coating 102 has a combination of properties that enables itto penetrate or access such cavities. Dielectric coating 102 attaches tothe component and printed wiring board surfaces forming such cavitieswhen applied via air atomizing techniques, as described above.

Although dielectric coating 102 sufficiently coats the component andboard surfaces that define cavities, such surfaces are the moredifficult surfaces to coat with conformal EMI shield 100. In one aspectof the invention, a non-electrically-conductive, high viscosity materialis applied to specific regions of printed circuit board 304 tofacilitate the coating of cavities on the printed circuit board. Thisaspect of the invention will be described with reference to FIGS. 9A–9D.FIG. 9A is a cross-sectional view of two components 302 mounted on aprinted wiring board 202. In this example, one cavity 900A is locatedbelow the bottom surface of raised component 914A while two additionalcavities are beneath the leads 906 of component 914B.

Those components or groups of components 914 that have or form suchcavities 900 with each other and/or printed wiring board 202 are coveredat least partially with a viscous, non-electrically-conductive fillermaterial 902. Filler material 902 bridges across the opening(s) of eachcavity 900 to cover, enclose, encapsulate and seal the cavity.Oftentimes, the cavities 900 are also at least partially infilled withfiller material 902. Referring to the exemplary application shown inFIG. 9A, for example, cavities 900A and 900B are infilled while cavity900C is not. Regardless of whether a cavity 900 is infilled, however, acoating of filler material 902 eliminates the requirement thatdielectric coating 102 penetrate cavities 900 to coat component andboard surfaces defining the cavity 900. In addition, filler material 902can also be applied to highly variable surfaces of printed circuit board304. Highly variable surfaces include surfaces having a surface tangentthat experiences substantial changes in value and/or abrupt changes insign over small regions.

Selective applications of filler material 902 converts the irregular andcratered printed circuit board surface to a contiguous surface havinggradual transitions due to the covering of cavities and the smoothing ofsharp and abrupt surfaces. In other words, a printed circuit board 304having filler material 902 applied thereto has a surface tangent thatdoes not change abruptly and which does not have cavities. Dielectriccoating 102, when applied to components covered with filler material 902will coat completely such components due to the contiguous, contouredsurface provided by filler material surface 912. Thus, filler material902 insures the successful insulation of printed circuit board 304 priorto the application of conductive coating 104.

Although the viscosity can vary, filler material 902 is preferablythixotropic, enabling it to be extruded into and over cavities 900 whilecovering the top, side and other surfaces of components 914. In oneembodiment, filler material 902 is an epoxy such as any epoxy from thefamily of Bisphenol-A epoxies mixed with an amine hardener. In oneparticular embodiment, filler material 902 is an EMCAST, CHIPSHIELD,3400–2500 and 3600 series epoxies available from Electronic Materials,Inc., Breckenridge, Colo. A thermally cured epoxy is preferred due tothe inability to directly apply UV radiation to filler material 902 thatis disposed in cavities 900 due to shadows cast by the components.

In another embodiment, a latex based non-electrically conductivecoating, such as HumiSeal TS300 epoxy, sold under the tradenameTEMPSEAL, available from HumiSeal, Woodside, N.Y. In contrast to theBisphenol-A epoxies noted above, HumiSeal TS300 can be removed fromprinted circuit board 304 by manually peeling it from the componentsurfaces. In another embodiment, the epoxy ABLEBOND 9349K available fromTra-Con, Inc. is utilized as filler material 902. This epoxy is a gray,two-part epoxy manufactured with glass bead spacers to control the bondline thickness.

It should be understood that due to variations in material, surfacecavity configuration, application technique or a combination thereof,filler material 902 may cure with one or more voids. For example,referring to FIG. 9A, filler material 902 did not bridge completelyacross neighboring leads 906 in certain locations, forming voids 904Aand 904B. FIG. 9B is a top view of void 904A. As shown, filler material902 fills the cavity 900A between and below neighboring leads 906. Void904A extends into the space between leads 906, exposing a portion 908 oflead 906A. If conductive coating 104 were to be applied to fillersurface 912, void 904A would result in a short circuit of the exposedcomponent lead 908. Thus, although such a circumstance can be eliminatedthrough controlled processes, dielectric coating 102 is preferablyapplied to all surfaces of printed circuit board 304, including surface912 of filler material 902. This insures that voids 904, if any, arecompletely insulated from the subsequently applied conductive coating104. FIG. 9C is a cross-sectional view of the components shown in FIG.9A, with a dielectric coating 104 applied to filler surface 912 offiller material 902 and the surface of printed wiring board 202. Asshown in FIG. 9C, dielectric coating 104 completely covers surface 912of filler material 902, including voids 904. As used herein, such voids,when coated with dielectric coating 104, are referred to as insulatedvoids 910. Application of conductive coating 104, as shown in FIG. 9D,results in a conformal EMI shield 100 that completely covers, whilebeing electrically isolated from, printed circuit board 304.

It should be understood that the method for applying filler material 902is a function of the selected material and specified by themanufacturer. Other operations may be included as well. For example, toavoid the formation of air pockets within or below the filler material902 adjacent to components, the surface to be coated is subjected tonegative pressure prior to the application of filler material 902. Thiseliminates the possibility of trapping air where it could corrodecomponent surfaces. It should also be understood that multiple fillermaterials 902 can be incorporated into an EMI protected printed circuitboard, for example, when different filler materials have differentcombinations of viscosity, thermal conduction and other properties eachsuitable for coating different components.

7. Decoupling Circuit

As noted, electrical noise is a significant problem that must be managedin today's high speed circuit boards. Electrical noise serves asinterference to transmitted data as well as power signals. Inparticular, interfering signals, referred to herein as conductedinterfering signals, can travel over leads or wires terminating on aprinted circuit board. Further, electromagnetic fields, commonlygenerated around leads attached to the printed circuit board, can becapacitively, magnetically and/or electromagnetically coupled toconductors on the printed circuit board, inducing additional interferingsignals on the printed circuit board. Such induced signals, which can bein the form of fields or currents, are referred to herein as inducedinterfering signals.

The manner in which leads can be connected to a printed circuit board isillustrated in FIG. 3. In exemplary printed circuit board 304, powerfeed-through post 308 is mounted on printed wiring board 202 forreceiving power from an external source. Signal connector 312, which isalso mounted on printed circuit board 304, is connected to leads orwires connected to an external device to transfer signals to and/or fromprinted circuit board 304. Oftentimes, a printed circuit board has anumber of such signal connectors or similar devices for connectingsignal leads to the printed circuit board. For ease of reference, theseand other signal and power posts and connectors, and other devices usedto transfer power and signals to/from a printed circuit board, arereferred to herein as interconnect posts.

In this aspect of the present invention, a noise suppressor arrangementutilizes conformal EMI shield 100 to eliminate interfering signalsinduced on a printed circuit board. Specifically, the noise suppressor,referred to as a conformal coating noise suppressor, is implemented as areceiver loop formed by a portion of conformal EMI shield 100 and aconductive path through which induced interfering signals travel. Thesuppressor receiver loop is arranged such that induced canceling signalstravel through the shared conductive path in a direction opposite to thedirection in which the induced interfering signals travel, providing thenoted canceling effect.

In the embodiment described below, this arrangement is implemented in adecoupling circuit that also comprises an electrical filter thatdecouples conducted interfering signals received at the printed circuitboard interconnect post. Induced interfering signals can be induced in areceiver loop formed in part by electrical filter components. Thesuppressor receiver loop generates noise suppressing signals that cancelinterfering signals induced in the electrical filter circuit by externalfields.

One embodiment of a decoupling circuit of the present invention isillustrated in FIG. 16A. Decoupling circuit 1600 primarily comprises aconformal coating noise suppressor 1602 and an electrical filter circuit1604. Electrical filter circuit 1604 reduces conducted interferingsignals 1608 that travel over power input or signal input and outputleads connected to an interconnect post 1004 on printed circuit board304. As shown in FIG. 16A, transmitted signals 1606 and conductedinterfering signals 1608 are decoupled by electrical filter 1604.Electrical filter circuit 1604 shunts conducted interfering signal 1608to ground and forwards only transmitted signal 1606 to the destinationcircuit on printed circuit board 304.

In FIG. 16D, induced interfering signals 1610 are generated on printedcircuit board 304 and interfere with transmitted signal 1606. Conformalcoating noise suppressor 1602 prevents such induced interfering signals1610 from reaching the destination circuit by utilizing conformal EMIshield 100 to generate a cancellation signal 1612, as described indetail below. Thus, decoupling circuit 1600 decouples conductedinterfering signals 1608 from transmitted signals 1606 and reducesinterfering signals 1610 induced in electrical filter 1603 by magneticand electromagnetic coupling of external fields.

FIG. 16A is a side cross-sectional view of a decoupling circuit 1600 inaccordance with one embodiment of the present invention. It should beunderstood that printed wiring board 202 can have any number of layerssuitable for a given application. However, for ease of illustration,only the portion of printed wiring board 202 that includes ground plane404 is shown in FIG. 16A.

In this exemplary implementation, decoupling circuit 1600 is mounted onprinted wiring board 202 to filter signals received on lead 1616connected to interconnect post 1604. Interconnect post 1604 is mountedon top surface 322 of printed wiring board 202 and extends through a viain printed wiring board 202 to electrically connect to a surface trace1630 disposed on bottom surface 320 of printed circuit board 304. Asshown in FIG. 16A, this surface trace 1630 leads to a destinationcircuit (not shown) mounted elsewhere on printed wiring board 202.

A ground land 1614 is mounted on top surface 322 of printed wiring board202. Ground land 1614 is connected to a ground via 406 which, in turn,is connected to ground plane 404, providing a continuous electrical pathfrom ground land 1614 to ground potential. In the exemplaryimplementation, ground via 406 extends through printed wiring board 202and is accessible from bottom surface 320 of printed wiring board 202.Alternatively, ground via 406 can terminate at ground plane 404 and notextend through printed wiring board 202. It should be further understoodthat ground land 1614 can have any dimensions suitable for a givenapplication and which provides a surface area sufficient to adhere toconductive coating 104.

Electrical filter 1604 comprises one or more lumped, distributed ordissipative elements selected and arranged to provide a predeterminedsignal filtering capability. Electrical filter circuits are well-knownin the art and are not described further herein. In this particularapplication, interconnect post 1604 receives power from an externalpower source, and electrical filter 1604 comprises a two-terminalsurface-mounted capacitor 1612 connected between interconnect post 1604and ground land 1614 to decouple high frequency conducted interferencesignals 1608 from the received power signals.

As shown in FIG. 16A, capacitor 1612 is electrically connected tointerconnect post 1604 and ground land 1614 through surface traces 1630disposed on top surface 322 of printed wiring board 202. Interconnectpost 1604 is coupled to a surface trace 1630 that leads to capacitor1612. In the illustrative embodiment, the portion of interconnect post1604 that extends from printed wiring board 202 is flared to provide asufficiently large surface for abutting surface traces 1630 and, as willbe noted below, to provide a shoulder for receiving dielectric coating102. Similarly, ground land 1614 is connected to the ground terminal ofcapacitor 1612 through a surface trace 1630, providing a path to groundpotential through ground via 406 and ground plane 404.

Portions of printed circuit board 304, including decoupling circuit1600, are coated with conformal EMI shield 100. Conformal EMI shield100, as noted, comprises dielectric coating 102 and conformal coating104. Dielectric coating 102 is conformingly adhered to surface-mountedcapacitor 1612 and surface traces 1630. Gaps or openings havingpredetermined locations and dimensions are provided in dielectriccoating 102 to leave exposed interconnect post 1604 and ground land1614. As shown in FIG. 16A, dielectric coating 102 is applied to ashoulder region of interconnect post 1604, ensuring that surface trace1630 connecting interconnect post 1604 and capacitor 1612 is completelyinsulated.

Conductive coating 104 is insulated from electrical filter 1604 andsurface traces 1630 by dielectric coating 102. Conductive coating 104,however, is electrically connected to ground land 1614 through the notedgap in dielectric coating 102. In addition, conductive coating 104 ismasked or otherwise prevented from contacting interconnect post 1604. Asa result, interconnect post 1604 remains exposed after application ofconformal coating 104.

FIG. 16B is the same cross-sectional view as that depicted in FIG. 16A.However, certain portions of FIG. 16B are emphasized while others areshown in phantom. Specifically, a receiver loop in which interferingsignals 1610 can be induced due to capacitive (electrostatic), magneticand/or electromagnetic coupling with external fields is highlighted inFIG. 16B. This receiver loop, referred to as filter receiver ioop 1618,is formed by surface-mounted capacitor 1612, interconnect post 1604,ground via 406, ground plane 404, and interconnecting surface traces1630. Interfering signals 1610 induced in filter receiver loop 1618 cantravel to other parts of printed circuit board 304 over surface orinternal traces, as well as through conductive coating 104.

In certain embodiments of the invention, the configuration of electricalfilter 1604 and the arrangement of the selected elements of printedcircuit board 304 are determined so as to minimize the field couplingcharacteristics of filter receiver loop 1618. In the embodimentillustrated in FIGS. 16A–16D, surface-mounted capacitor 1612 is locatedimmediately adjacent to interconnect post 1604. Similarly, ground land1614 is located immediately adjacent to the ground terminal of capacitor1612. Preferably, the relative proximity of these components is limitedto that which the manufacturing process can accommodate. This reducesthe area encompassed by filter receiver loop 1618, resulting in aconcomitant reduction in potential coupling with external fields.

It is a well-understood phenomenon that the parasitic inductance of acapacitor increases with the physical size of the capacitor. Thus, whenthe particular application permits, it is preferred to use capacitorswith relatively smaller physical dimensions. Since the capacitance ofsuch capacitors is likewise less than their larger counterparts, morethan one smaller capacitor may be implemented in a series or parallelarrangement to achieve a desired filtering characteristic that couldotherwise be provided by a single, larger capacitor. For example, toprovide 0.4-μf of capacitance, it would be preferable to implement two0.2-μf capacitors in parallel rather than a single 0.4-μf capacitor.Such an arrangement provides the desired capacitance while reducing theparasitic inductance.

FIG. 16C is a cross-sectional view of the same circuit illustrated inFIGS. 16A and 16B. Decoupling circuit 1600 comprises, as noted, aconformal coating noise suppressor 1602. In accordance with this aspectof the invention, conformal coating noise suppressor 1602 is asuppressor receiver ioop 1620 formed in part by EMI conformal shield 100(not shown in FIG. 16C) and which includes a conductive path throughwhich induced interfering signals 1610 travel. In FIG. 16C, a noisesuppressor receiver loop 1618 is highlighted while the remainingportions of the figure are shown in phantom.

As highlighted in FIG. 16C, suppressor receiver loop 1620 is a receiverloop formed of conductive coating 104, interconnect post 1604,surface-mounted capacitor 1612, ground land 1614 and interconnectingsurface traces 1630. Thus, suppressor receiver loop 1620 and filterreceiver loop 1618 (highlighted in FIG. 16B share an electrical pathbetween interconnect post 1604 and ground land 1614 throughsurface-mounted capacitor 1612. That is, surface-mounted capacitor 1612,interconnect post 1604, ground land 1614 and interconnecting traces 1630form a conductive path common to receiver loops 1618 and 1620.

FIG. 16D is a cross-sectional view of the same circuit illustrated inFIGS. 16A through 16C. In FIG. 16D, the current induced in each receiverloop, that is, induced interfering signal 1610 and induced cancelingsignal 1622, is depicted with representative arrows illustrating therespective direction of travel. External fields magnetically andelectromagnetically coupled to filter receiver loop 1618 are alsocoupled to the conformal coating noise suppressor receiver loop 1620.Induced interfering signal 1610 travels in a counter-clockwise directionthrough filter receiver ioop 1618. Similarly, induced canceling signal1622 travels in a counter-clockwise direction around suppressor receiverloop 1620. The induced signals have the same direction of travel aroundtheir respective receiver ioops due to receiver ioops 1618 and 1620having the same relative relationship with external fields. Because thecurrent induced in the two receiver loops travel in opposite directionsthrough a common conductive path, signal 1622 generated by noisesuppressor 1602 cancels interfering signals 1610 induced in filterreceiver loop 1618.

It should be appreciated that the magnitude of induced canceling current1622 can be determined empirically and can be controlled by adjustingthe area enclosed by receiver loop 1620, the thickness of conformalconductive coating 104 which covers surface-mounted capacitor 1612, etc.Thus, the magnitude of induced canceling current 1622 can be controlledto achieve a desired canceling effect on induced interfering current1610.

As noted, external fields that surround lead 1616 can capacitvely induceunwanted fields in conductive coating 104. Conductive coating 104, asnoted, surrounds interconnect post 1604, and provides a conductivesurface over which such electrostatically-induced fields can travel. Inanother aspect of the invention, a grounded coaxial compartment encasessurface-mounted capacitor 1612. Such a compartment provides an immediatepath to ground for any noise induced in conductive coating 104, such asby fields around leads 1616 and interconnect post 1604.

One exemplary embodiment of this aspect of the invention is illustratedin FIG. 17, in which a front cross-sectional view of decoupling circuit1600 is shown. This cross-sectional view is taken orthogonal to the viewillustrated in FIG. 16B, as shown by the cross-section lines depictedtherein. In this embodiment, additional ground lands 1614 are radiallyspaced around the periphery of surface-mounted capacitor 1612. Eachground land 1614 is connected to ground plane 404 in manner similar tothat noted above. Thus, printed circuit board 304 provides a pluralityof ground vias 406 located immediately adjacent to and radially spacedaround surface-mounted capacitor 1612. As with the embodimentillustrated in FIG. 16B, conductive coating 104 is electricallyconnected to each ground land 1614. In such an arrangement, a portion ofconductive coating 104, the plurality of ground lands 1614, and groundplane 404 form a grounded coaxial compartment 1702 encasingsurface-mounted capacitor 1612. Grounded compartment 1702 shunts fieldsand currents traveling over conductive coating 104 to ground, preventingtheir traveling to other regions of printed circuit board 304.

Thus, decoupling circuit 1600 reduces interfering signals that can enterelectrical systems through power-line inputs or through signal input andoutput lines while also reducing the effects of interfering signalsgenerated as a result of electrostatic, magnetic and electromagneticcoupling of filter receiver loop 1618.

FIGS. 18A–18C are top views of a portion of printed circuit board 304that includes decoupling circuit 1600. Collectively, these figuresillustrate the manufacturing process that can be implemented to createconformal coating noise suppressor 1602 of the present invention. FIG.18A is a top view of printed circuit board 304 prior to application ofconformal EMI shield 100. In FIG. 18A, top surfaces of ground lands1614, interconnect post 1604 and surface-mounted capacitor 1612 areillustrated. As shown, capacitor 1612 is a two-terminal capacitor withone terminal connected to interconnect post 1604 and the opposingterminal connected to ground land 1614 (not shown in FIG. 18A).

FIG. 18B is a top view of printed circuit board 304 shown in FIG. 18Asubsequent to the application of dielectric coating 102 to printedcircuit board top surface 322. As shown in FIG. 18B, ground lands 1614remain visible through openings 1802 in dielectric coating 104.Similarly, interconnect post 1604 remains exposed through an opening1804 around interconnect post 1604. Such openings 1802 and 1804 have, asnoted, predetermined locations and dimensions.

FIG. 18C is a top view of printed circuit board 304 shown in FIGS. 18Aand 18B subsequent to the application of conductive coating 104 to thesurfaces of printed circuit board 304. As shown, only interconnect post1604 has been masked and remains exposed after conformal EMI shield 100is applied to printed circuit board 304.

8. Manufacturing of Printed Circuit Board with Conformal EMI Shield

FIG. 10 is a flow chart of the primary operations performed inaccordance with one embodiment of the present invention to form aprinted circuit board with a conformal EMI shield 100 adhered tosurfaces thereof. Printed circuit board 304 is manufactured at blocks1002 and 1004. At block 1002 printed wiring board 202 is formed. Printedwiring board 202 is constructed as described above. For example, printedcircuit board 202 typically includes ground lands in predeterminedlocations to be connected to conductive layer 104. Printed wiring board202 may also include a series of ground vias located along the peripheryof printed wiring board 202. As noted, such ground vias provideelectrical continuity of conformal EMI shield 100 applied to the top andbottom surfaces 322, 320 of printed circuit board 304. At block 1004,printed wiring board 202 is populated with components to form one ormore circuits, the sum of which is referred to herein as printed circuitboard 304.

Printed circuit board 304 is then prepared for application of conformalEMI shield 100 at block 1006. This may include any preparatoryactivities required or desired to facilitate the application ofconformal EMI shield 100 to printed circuit board 304. For example,soldering residues may interfere with the ability of dielectric coating104 to adhere to printed circuit board 304. Preferably, such solderingresidues are washed off printed circuit board 304 at block 1006.

At block 1008 highly viscous filler material 902 is optionally appliedto predetermined components to fill and cover cavities thereof, as wellas cavities between neighboring components and between components andprinted wiring board 202. Filler material 902 may cover or encapsulatethe component(s) or group of components, as noted above. Filler material902 can be applied using any well-known extrusion or other techniquethat will not damage the covered components 302.

At block 1010 one or more component covers 802 are optionally mounted onprinted wiring board 202 to cover certain, predetermined components. Asnoted, such components can include those that are fragile or expensive.Component covers 802 provide for future access to such components withminimal interference from conformal EMI shield 100.

Dielectric and conductive coatings 102, 104 are applied at blocks 1012and 1014, respectively. A number of exemplary embodiments of dielectriccoating 102 and conductive coating 104 have been described above. In allsuch embodiments, coatings 102, 104 are applied as a liquid. As such,dielectric coating 102 and conductive coating 104 are generally referredto as liquid coatings when they are applied to surfaces of printedcircuit board 304. Exemplary application techniques for applying suchliquid coatings include, for example, conventional atomization spray,dipping and painting techniques.

Dielectric coating 102 and conductive coating 104 are likely applied topredetermine regions of printed circuit board 304. In one embodiment,this is achieved by masking the printed circuit board 304.Alternatively, the coatings 102, 104 can be selectively applied to thedesired regions of printed circuit board 304 using a precision sprayapplication technique. Masking may be first applied prior to theapplication of dielectric coating 102. Then, after dielectric coating102 has cured, any exposed masking unique to dielectric coating 102 isthen removed. Printed circuit board 304 can then be re-masked asnecessary to prevent conductive coating 104 from contacting connectorcontacts, etc., as noted above. Conductive coating 104 is then appliedto printed circuit board 304, followed by the removal of the exposedportions of this masking from printed circuit board 304.

Once applied, dielectric and conductive coatings 102,104 are cured,causing coatings 102,104 to transition from a liquid to solid state, andadhere to the surface of printed circuit board 304. In certainembodiments, coatings 102, 104 are applied in multiple layers, with eachsuch layer being cured prior to application of a subsequent layer.Curing time and temperature are specified by the manufacturer of thecomponent materials of each coating 102,104, and may vary betweendielectric coating 102 and conductive coating 104 as well as betweendifferent embodiments of each coating.

Dielectric coating 102 and conductive coating 104 can be cured whileprinted circuit board 304 is oriented horizontally. That is, the planeof the board is oriented in an approximate horizontal position withcomponents mounted on the top and/or bottom surface 322, 320 thereof.Typically, the liquid coatings are cured in a fixture that exposes theprinted circuit board 304 to heat, ultraviolet light, humidity or othercuring agent to accommodate particular material and curing requirements.

However, when in their liquid state, dielectric coating 102 andconductive coating 104 may have a tendency to flow off vertical surfacesof components, leads, etc., on printed circuit board 304 in response tothe pull of gravity. For components mounted on printed circuit board topsurface 322, the applied liquid coatings could accumulate or pool at lowpoints on the board. For components mounted on printed circuit boardbottom surface 320, the applied liquid coatings could simply drip off ofthe component before they are cured. Such an event may more readilyoccur with certain embodiments of dielectric coating 102 and conductivecoating 104 which become less viscous and, therefore, more likely torun, as their ambient temperature increases. Such run-off can beparticularly problematic should it cause voids to develop in dielectriccoating 102 which serves as a layer of insulation between components andtraces of printed circuit board 304 and conductive coating 104.

In accordance with one aspect of the invention, printed circuit board304 is manipulated while the applied liquid coating is cured to reduceor eliminate potential adverse and unintended effects of gravity duringthe curing process. In one particular embodiment described below,printed circuit board 304 is periodically rotated during curing.Specifically, printed circuit board 304 is continually rotated about oneor more horizontal axes extending through a plane defined by printedcircuit board 304. Such rotations cause the gravitational force vectorto continuously change and average to approximately zero over the timeprinted circuit board 304 is rotated. This causes the applied liquidcoatings to essentially remain in a stationary position; that is, theyare prevented from flowing in response to gravity. Importantly, thisprevents the formation of voids in dielectric coating 102, insuringdielectric coating 102 provides a continuous layer of insulationwherever it is applied.

FIG. 19 is a perspective view of a curing unit for use in a processingline for manufacturing printed circuit boards. Such curing equipment iscommonly combined with other stand-alone conveyor-to-conveyor equipmentto form an SMT (surface mounted technology) process line. Commonly, suchstand-alone systems are designed in accordance with interface standardsestablished by SMEMA (Surface Mount Equipment Manufacturer'sAssociation). These standards provide equipment interface specificationsfor single board transfer manufacturing equipment of surface-mountedprinted circuit boards, enabling them to be integrated into a larger,continuous belt, process line. A typical SMT process line could include,for example, equipment that mounts components on surfaces of a printedwiring board 202, equipment that dispenses liquid coatings on a printedwiring board 202 or printed circuit board 304, and equipment that curesthe applied liquid coatings.

Curing unit 1900 includes a base 1908 supporting a conveyor belt 1906that travels from one end of the fixture 1900 to the other. A hood 1910covers conveyor belt 1906, defining an operational region of curing unit1900, referred to as a tunnel 1902. The speed at which conveyor belt1906 transports a printed circuit board 304 through tunnel 1902 and,thus, the curing time, is controlled through control panel 1904. Otherparameters such as the curing temperature can also be set by an operatorthrough control panel 1904. It should be understood that curing unitscan implement different curing environments such as a high temperature,infra-red (IR), ultraviolet (UV) or humidity chamber curingenvironments. For example, in one embodiment of the present invention,curing unit 1900 is a 2000-series curing oven commercially availablethrough Precision Valve and Automation, Halfmoon, N.Y.

Generally, the present invention can be implemented in any apparatusthat can effect a controlled rotation of printed circuit board 304 aboutat least one horizontal axis. The apparatus is preferably capable ofoperating in the curing environment implemented in the process line. Twoexemplary embodiments of an apparatus of the present invention aredepicted in FIGS. 20 and 21 and described below.

FIG. 20 is a perspective view of a rotation cage 2000 suitable forrotating printed circuit board 304. Rotation cage 2000 is configured toimplement the desired rotation of printed circuit board 304 whileminimizing interference with the exposure of printed circuit board 304to the curing agent. In the example shown in FIG. 20, a frame structure2002 with apertures 2010 on five of the six sides is provided. Apertures2010 have dimensions sufficient to not interfere with the convection ofheat, the impinging of UV light or exposure to other curing agents.

A clamp 2008 is located at the distal end of a shaft extending from amotor 2006 that rotates printed circuit board 304 in accordance withcommands or instructions entered or programmed into rotation cage 2000through control panel 2004. For example, in one embodiment, theparameters that could be specified by the operator include the rate ofrotation, the orientation at which the printed circuit board is rotated,the time interval between successive rotations, the quantity ofrotations, among others. As one of ordinary skill in the art would findapparent, such parameters are specified based on the embodiments ofdielectric coating 102 and conductive coating 104 that are implemented,the thickness of the liquid coating that is being cured, etc.

The dimensions of rotation cage 2000 is preferably such thatmodifications to curing unit 1900 are not necessary to accommodate useof rotation cage 1900. For example, rotation cage 2000 is preferablyconfigured to be capable of being placed on conveyor belt 1906 andtravel through tunnel 1902 of curing oven 1900. Similarly, the weight ofrotation cage 2000 is preferably within the weight tolerances specifiedfor devices to be cured in curing unit 1900. It should be understoodthat other embodiments of rotation cage 1900 that satisfy the above andother criteria could also be implemented and that such embodiments areconsidered to be within the scope of the present invention.

In an alternative embodiment, the apparatus that effects the rotation ofprinted circuit board 304 is temporarily or permanently integrated intocuring unit 1900. For example, a robotic arm such as that illustrated inFIG. 21 could be mounted within curing unit 1900. Such an articulatingarm 2100 could be programmed to lift printed circuit board 304 fromconveyor belt 1906, rotate it about one or more axes while the appliedliquid coating 102,104 is cured, and return it to conveyor belt 1906 foradvancement to the next processing line equipment.

9. Closing

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. For example, it should be appreciatedthat the present invention can be implemented in any type of curingenvironment other than in conveyor-to-conveyor single board SMT curingequipment. It should also be appreciated, then, that the configurationof an apparatus that implements the present invention can vary toaccommodate the curing environment. Thus, the breadth and scope of thepresent invention should not be limited by any of the above-describedexemplary embodiments, but should be defined only in accordance with thefollowing claims and their equivalents.

1. An apparatus for rotating a printed circuit board (PCB) while it isbeing exposed to a curing agent in a curing environment of a curingunit, comprising: a frame structure dimensioned to be temporarilypositioned within the curing environment; an extension arm adapted todetachably secure the PCB; and a motor that imparts a controlledrotation of at least a portion of the extension arm to cause the PCBsecured to the distal end thereof to rotate about at least one axislocated in a plane defined by the PCB.
 2. The apparatus of claim 1,wherein the curing unit comprises a conveyor belt adapted to transferthe PCB through the curing environment, and wherein the apparatus isdimensioned to travel on the conveyor belt to be temporarily positionedin the curing environment.
 3. The apparatus of claim 1, wherein theapparatus is responsive to control inputs defining at least onecharacteristic of the rotation of the PCB.
 4. The apparatus of claim 3,wherein the at least one characteristic comprises a rate at which thePCB is rotated.
 5. The apparatus of claim 3, wherein the at least onecharacteristic comprises an orientation of the PCB during rotation. 6.The apparatus of claim 3, wherein the at least one characteristiccomprises a time interval between successive rotations of the PCB. 7.The apparatus of claim 3, wherein the at least one characteristiccomprises a quantity of rotations of the PCB.
 8. The apparatus of claim1, wherein the curing agent comprises one or more of a group consistingof: elevated temperature; infra-red (IR) light; ultra-violet (UV) light;and humidity.
 9. The apparatus of claim 1, wherein the apparatus furthercomprises: a control system operable to control at least one operationalparameter defining a characteristic of the rotation of the PCB.